Population Biology of the Introduced Rose-ringed Parakeet ...

Population Biology of the Introduced Rose-ringed
Parakeet Psittacula krameri in the UK
Thesis submitted by
Christopher John Butler
For examination for the degree of Doctor of Philosophy
University of Oxford
Department of Zoology
Edward Grey Institute of Field Ornithology
November 2003

Abstract
This study describes the population biology of feral Rose-ringed Parakeets
Psittacula krameri in the UK. Rose-ringed Parakeets are native to Africa and the Indian
sub-continent, where they are considered to be one of the most significant agricultural
pests of fruits and grains. They were introduced into the UK in 1969, and the population
slowly increased to an estimated 500 birds in 1983 and 1500 birds by 1996. Given the
increase in their population, Rose-ringed Parakeets have the potential to outcompete
native cavity-nesting species for nest sites as they begin nesting in late February, before
most native species. In addition, they could become a serious agricultural pest. Roost
counts conducted during the winters of 2000/01 and 2001/02 revealed that the population
was increasing at an average annual rate of approximately 25% to approximately 6000
individuals by 2002. Roosts were present throughout the year, and radiotelemetry
revealed that male parakeets returned to the roost during the breeding season. A
significant discriminant function was created that correctly classified 92.1% of cases
utilizing the date of introduction, the numbers of introduced parrots, and the area. Nests
were monitored from 2001-2003 and it was found that reproductive rates were much
higher than previously reported, with 1.9 ± 0.1 young fledging per nest. Although Rose-
ringed Parakeet numbers are increasing rapidly, there is no evidence that they are having
an effect upon native species, although this may change in the future. A binary logistic
regression was developed in order to sex parakeets in the hand, and to examine annual
and daily body mass change by sex. (Previously, it had not been possible to separate
immature males from females). Unexpectedly, neither sex gained mass during the winter,
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despite the presumed increase in starvation risk. Population models were constructed and
revealed that populations in the Greater London area were increasing at a rate of
approximately 30% per year, while populations at the Isle of Thanet were increasing at a
rate of only 15% per year. Parakeets were expanding their range at a rate of
approximately 0.4 km/yr in the Greater London area, but were not yet expanding their
range at the Isle of Thanet. In order to prevent these parakeets from increasing further, an
annual harvest of approximately 30% of the population would need to be carried out.
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Contents
Abstract……………………………………………………………………………. 2
Contents…………………………………………………………………………… 4
List of tables.…………………………………………………………..………….. 5
List of figures…………………………………….……………………………….. 10
Acknowledgements………………………………………………………………... 16
Chapter 1: Introduction…………………………………………………………….. 18
Chapter 2: Population estimates and roosting behaviour………………………….. 54
Chapter 3: Factors influencing Rose-ringed Parakeet naturalization probability…. 86
Chapter 4: Breeding success in the UK……………………………………………. 127
Chapter 5: Aging & sexing parakeets……………………………………………… 174
Chapter 6: Body mass regulation………………………………………………….. 209
Chapter 7: Population and spatial modeling……………………………………….. 241
Chapter 8: Concluding comments…………………………………………………. 290
Appendix 1: Breeding parrots of Britain…………………………………………... 303
Appendix 2: First nesting by Blue-crowned Parakeet in Britain………………….. 307
Appendix 3: List of published papers……………………………………………... 311
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List of tables
Table 2.1 A summary of roost high counts from 1995-1999 80
Table 2.2 Monthly roost counts from November 2000 – April 2002. 81
Table 2.3 A summary of the radiotelemetry results. 82
Table 2.4 A summary of radiotelemetry visits to the roosts. 83
Table 3.1 A summary of non-native breeding populations of Rose-ringed 115
Parakeets across the world
Table 3.2 A summary of the GLM for predicting Rose-ringed Parakeet 121
population size in countries where they have been introduced
(n = 18).
Table 3.3 A summary of the GLM for predicting Rose-ringed Parakeet 122
population size in countries where they have been introduced
created using backwards deletion.
Table 3.4 A summary of the binary logistic regression used to separate 123
countries where Rose-ringed Parakeets were successfully
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naturalized form countries where they failed to establish
self-sustaining populations.
Table 3.5 A summary of the binary logistic regression used to separate 124
countries where Rose-ringed Parakeets were successfully
naturalized from countries where they failed to establish
self-sustaining populations created using backwards deletion.
Table 3.6 A summary of the discriminant function used to separate 125
countries where Rose-ringed Parakeets were successfully
naturalized from countries where they failed to establish self-
sustaining populations.
Table 3.7 A summary of the discriminant function used to separate 126
countries where Rose-ringed Parakeets were successfully
naturalized from countries where they failed to establish self-
sustaining populations using backwards deletion
Table 4.1 A summary of previous studies on the fledging rates of Rose- 154
ringed Parakeets
Table 4.2 A summary of tree height, nest height, dbh, basal area, and 155
basal stem count in the four regions were parakeet nests were
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found
Table 4.3 Characteristics of trees occupied by breeding Rose-ringed 156
Parakeets in southern England during 2001-2003 and a tree 100 m
away in random direction, and univariate logistic regression
parameter estimates
Table 4.4 A summary of the genera, height, and dbh of trees that parakeets 157
nested in over the course of 2001-2003
Table 4.5 A summary of the numbers of trees used by Rose-ringed 158
Parakeets compared with the numbers of randomly chosen trees
Table 4.6 A summary of breeding success during 2001-2003, presented as 159
mean ± s.e
Table 4.7 A significant GLM model (n = 77, R 2 = 0.216, p < 0.001) for 160
predicting fledging success based on clutch size and the number
of years the nest cavity was occupied
Table 4.8 A significant GLM model (n = 77, R 2 = 0.107, p = 0.003) for 161
predicting clutch size based upon tree height
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Table 4.9 A comparison of fledging rates between three regions (SW 162
London, SE London and Coastal Kent) with those values
reported by Pithon and Dytham (1999a)
Table 5.1 Wing, tail, bill, and toe measurements for the four subspecies 192
of Psittacula krameri based on three published sources
(Forshaw 1989, Pithon 1998, Roselaar 1985).
Table 5.2 A summary of the biometrics for parakeets trapped in the UK 193
during this study
Table 5.3 Classification table for a binary logistic regression function 194
that uses only wing length, bill length, and the number of
completely yellow underwing coverts
Table 5.4 Logistic regression predicting gender based on wing length, tail 195
length, bill length, toe length, mass, and the number of
completely yellow underwing coverts
Table 5.5 Moult scores for primaries 1-10 on 21 birds that were 196
moulting when captured
Table 5.6 Date, ring number, and tarsus measurements (in mm) for 197
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41 parakeets
Table 6.1 A minimum adequate model for mass (R 2 = 0.332) 231
Table 6.2 A summary of recaptures and their mass 232
Table 7.1 A summary of the Leslie matrices used in the population 274
modelling
Table 7.2 A comparison of life table figures for the two UK populations 275
Table 7.3 Results from population wave formulas 276
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List of figures
Figure 1.1 The range of the two African subspecies of Rose-ringed Parakeet, 52
P. k. krameri and P. k. parvirostris.
Figure 1.2 The range of the two Asian subspecies of Rose-ringed Parakeet, 53
P. k. borealis and P. k. manillensis
Figure 2.1 The locations of the five major roosts 84
Figure 2.2 Average (± stdev) roost counts at Lewisham Crematorium 85
for the period December 2000 to October 2002
Figure 4.1 Location of nests in the Greater London area 163
Figure 4.2 Location of nests at the Isle of Thanet 164
Figure 4.3 A summary of the percentage of nests that were reused from the 165
previous year
Figure 4.4 A histogram of first egg dates 166
Figure 4.5 A histogram of clutch size 167
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Figure 4.6 A histogram of the number of chicks fledged 168
Figure 4.7 A chart showing that the number of young fledged was higher 169
when parakeets reused a nest cavity (n = 35)
Figure 4.8 The percentage of nests that were reused in successive years 170
relative to the number of young fledged (n = 35)
Figure 5.1 A comparison of the number of yellow underwing coverts 198
Figure 5.2 Locations in the Greater London area where Rose-ringed 199
Parakeets were ringed
Figure 5.3 Locations at the Isle of Thanet, Kent, where Rose-ringed 200
Parakeets were ringed
Figure 5.4 Photographs of iris colour in adult males 201
Figure 5.5 The yellowish edging on primary feathers that supposedly 202
indicates an immature bird (Roselaar 1985) can often be seen
on adult male birds
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Figure 5.6 A photograph of a recently fledged parakeet showing the 203
primaries and the secondaries
Figure 5.7 A photograph of primaries and secondaries on a one-year old 204
parakeet
Figure 5.8 A photograph of primaries and secondaries on a two-year old 205
male parakeet
Figure 5.9 A photograph of the primaries and secondaries of an adult male 206
parakeet (e.g. >3 yrs old)
Figure 6.1 Locations in the Greater London area where Rose-ringed 233
Parakeets were ringed
Figure 6.2 Locations at the Isle of Thanet, Kent, where Rose-ringed 234
Parakeets were caught
Figure 6.3 Mass changes throughout the year (n = 292) 235
Figure 6.4 Average mass for each month (n = 292) 236
Figure 6.5 A bar graph of mass by month for each predicted sex (n = 144 237
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males and 87 females; insufficient information was gathered to
predict the sex of the other birds)
Figure 6.6 A bar graph of mass by month for mature males (those with a 238
pink-coloured neck ring) and immature males (those that lack
a pink-coloured neck ring)
Figure 6.7 A scatterplot of mass and time of day 239
Figure 6.8 A histogram of capture times (n = 158) at locations where 240
mist netting was carried out throughout the day
Figure 7.1 A chart of the log population growth for Rose-ringed Parakeets 277
in the Greater London area and the Isle of Thanet
Figure 7.2 A chart of the population models for Greater London 278
Figure 7.3 A chart of the various models for the Isle of the Thanet 279
Figure 7.4 A chart demonstrating the exponential growth potential of 280
Rose-Ringed Parakeets in the Greater London area
Figure 7.5 A chart demonstrating the exponential growth potential of 281
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Rose-ringed Parakeets at the Isle of Thanet
Figure 7.6 Demographic sensitivities and elasticities for the Leslie matrices 282
used to model the population in the Greater London area and
the Isle of Thanet area
Figure 7.7 A graph of the projected effect of six management regimes on the 283
population size of Rose-ringed Parakeets in the Greater London
area
Figure 7.8 A graph of the projected effect of six management regimes on the 284
population size of Rose-ringed Parakeets in the Isle of Thanet area
Figure 7.9 A map of the data (courtesy of the BTO) included in the 1988-91 285
Breeding Bird Atlas (Gibbons et al. 1993)
Figure 7.10 A map of sightings where parakeets were found breeding or were 286
seen repeatedly during 2000-03
Figure 7.11 A graph of the number of number of squares occupied over time 287
Figure 7.12 A histogram of dispersal distances (α) based on new breeding 288
records from county bird reports in the Greater London area
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Figure 7.13 Wavefront sensitivities and elasticities for the Greater London 289
and Isle of Thanet populations
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Acknowledgements
Funding for my research was provided by grants from the Royal Society for the
Protection of Birds and the American Ornithologists’ Union.
I thank the Runnymede Ringing Group and the Rye Meads Ringing Group for
their guidance and assistance in capturing Rose-ringed Parakeets. Particular thanks go to
D. Griffin, D. Ross, P. Davies, and P. Roper for assistance and guidance ringing
Parakeets and for their willingness to share their data.
A special thanks to everyone who allowed me to ring parakeets in their gardens,
including J. Witt, F. and K. Hammer, A. Wenham, M. Barnes, J. Phillips, P. Isaacks, D.
Greenfield, Mr. & Mrs. Ames, R. and L. Nathan, J. and A. Helm, R. Bavin, C. Valente
and G. Schurter, and M. Hayward.
Many thanks to all the people who responded to my request for sightings by
entering information into my Project Parakeet website. Thanks also to all the people who
shared information on parakeet nest sites.
Special thanks to G. Hazlehurst for sharing his information on roost counts and
behaviour of parakeets at Lewisham Crematorium, as well as for our many interesting
discussions.
My graduate committee, C. Perrins, A. Gosler, and W. Cresswell have been
tremendously helpful in improving both the design of my research as well as the quality
of my manuscripts. I particularly thank my advisors, C. Perrins and A. Gosler for all their
generous help, encouragement and good advice. My labmates also provided helpful
advice and critiques. Many thanks to J. Lindsell, K. Evans, R. MacLeod, and J. Neto for
16

all their assistance. I am also grateful to the other people in the EGI and my other
colleagues who have made my time as a D.Phil student both enjoyable and productive.
Finally, Kristie Butler has spent countless hours out in the field with me, ringing
parakeets, climbing trees to look at nests, safely navigating us through the roads of
Greater London, and (not infrequently) sitting in traffic on the M25. This thesis would
not have been possible without her hard work, intelligent suggestions, unfailing good
humour, and companionship.
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Chapter 1:
Introduction
Introduced species are widely considered to be a serious ecological problem
(Bury and Luckenbach 1976, Baker 1990, Temple 1992, Manchester and Bullock 2000).
Gilpin (1990) says, “…exotic invaders have been the single greatest cause of species
extinction ..“ They are often cited as causing serious damage to landscapes and
ecosystems, particularly on islands (Savidge 1987, Williamson and Fitter 1996, Daehler
and Gordon 1997) but introduced species are a worldwide phenomenon. Indeed,
introduced species have even been recorded in Antarctica (Clayton et al. 1997).
Before a serious discussion of introduced species begins, however, it is useful to
define the terms that are often used. An “introduced species” is one that has been
deliberately or accidentally set free in a location where it is not native (Williamson and
Fitter 1996). “Non-native”, “non-indigenous”, “alien” and “exotic” are often used as
synonyms (Manchester and Bullock 2000). “Feral” species are those which were once
domesticated (or cultivated), but which escaped or were set free and are now living in a
wild state in a region to which they are not native (Manchester and Bullock 2000). It is
important to note that these terms do not necessarily imply that the introduced population
is self-sustaining. Once an introduced species has established a self-sustaining population
it is referred to as “naturalized” (Manchester and Bullock 2000). Naturalized species are
considered “invasive” if they are able to expand into new areas (Usher 1986, Usher et al.
1988). Often such invasive species are considered “pests” (Williamson 1993), although
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the meaning of “pest” varies from author to author, with many authors considering it to
refer only to the economic damage an invasive species does (Williamson and Fitter 1996)
rather than to the ecological damage it does (Usher 1986).
The object of this chapter is to examine why introduced species are released
(either intentionally or unintentionally), as well as the ways in which they may have a
negative impact. In addition, hypotheses about the characteristics of successful invasive
species are considered. Finally, the case study of a species successfully become
naturalized in many countries, the Rose-ringed Parakeet Psittacula krameri, is
introduced.
Causes of introduction
There are four reasons why a species may be introduced into a new location.
These are as follows:
1.) Deliberate release by acclimatization societies. Such societies were common
during the nineteenth century and were responsible for the introduction of numerous
plants and animals (Case 1996). Many of the introductions failed, but some were
successful and resulted in naturalized populations. Perhaps the most famous example
involves the release of European Starlings Sturnus vulgaris into New York during 1890-
91. From these 80-100 birds, the population has grown to approximately 200 million
(Cabe 1993).
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2.) Deliberate release for food and other products. In Britain, many plant species have
been deliberately introduced for crops or herbal medicines (Usher 1986). Fourteen crops
definitely have naturalized populations and another 22 may have naturalized as well
(Williamson and Fitter 1996).
Gamebirds are frequently introduced into areas where they are not native to
provide for hunting opportunities (Owen 1990, Blackburn and Duncan 2001, Duncan et
al. 2001, Duncan and Blackburn 2002). Similarly, fish are often introduced into new
areas to provide more opportunities for anglers (Hecnar and McCloskey 1997). Even
amphibians such as the Bullfrog Rana catesbiana have been introduced into new regions
for food (Bury and Luckenbach 1976).
3.) Accidental releases. Often, pets will escape from their owners and will thus be
introduced into a new region. These accidental introductions have resulted in the
establishment of numerous naturalized populations. In California (USA), for example,
seven species of amphibians have escaped from captivity and developed self-sustaining
populations (Bury and Luckenbach 1976). Monk Parakeets Myiopsitta monachus have
escaped from captivity in a number of cities around the world and their naturalized
populations are steadily increasing, even in cities as cold as Chicago, Illinois (USA),
where they have survived temperatures down to -33°C (Spreyer and Bucher 1998).
Occasionally, animals escape from zoos and establish naturalized populations.
One example of this is the Himalayan Porcupine Hystrix brachyura, which escaped from
the Pine Valley Wildlife Park near Okehampton in Devon in 1969 and began reproducing
successfully in the wild (Baker 1990).
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Fur farms, too, have accidentally introduced species into the wild. In the UK, the
most famous examples include the Mink Mustela vison (first imported in 1929 and
escaped soon after), the Muskrat Ondatra zibethicus (first imported in 1927 and escaped
soon thereafter), and the Coypu Myocastor coypus (first imported in 1937 and soon
escaped). Although efforts to eliminate Muskrat and the Coypu from the UK have been
successful, Mink are still widespread (Baker 1990).
Boats, cars, and other means of human transport can also accidentally introduce
species. The Norway Rat Rattus norvegicus is believed to have been introduced into the
UK during 1728-29 by ships travelling from Russia (Baker 1990). Similarly, the Zebra
Mussel Dreissena polymorpha was inadvertently introduced into North America through
ballast water discharged by an ocean-going vessel (Griffiths et al. 1991). Brown Anoles
Anolis sagrei in Florida and southern Georgia (USA) are extending their range north and
it is believed inadvertently being transported by cars and boats (Campbell 1996).
Finally, plant species can also be accidentally introduced when they escape from
gardens. One of the most famous examples in the UK is the rhododendron Rhododendron
ponticum. This species was introduced into the British Isles during the Victorian era,
when it was widely planted in ornamental gardens (Usher 1986). It is now widespread
throughout Britain and Ireland.
4.) Biological control. The growing number of invasive, pest species has led to a
recent surge of interest in using other non-native species to control their numbers. For
example, Purple Loosestrife Lythrum salicaria was introduced into North America during
the early nineteenth century and has degraded wetlands across the continent. Four insects
21

have been introduced into the US in an attempt to reduce the amount of Purple
Loosestrife. They include two weevils (Nanyophes marmoratus and Hylobius
transversovittatus) as well as two beetles (Galerucella calmariensis and Galerucella
pusilla) (Dech and Nosko 2002, McAvoy et al. 2002)
Detrimental effects of introduced species
Although introduced species are often vilified, not all introductions are
necessarily detrimental. The biological control agents mentioned above are successfully
reducing the quantity of Purple Loosestrife in the US (Fagan et al. 2002). Japanese
Cherry Muntingia calabura is used to reclaim mine spoils on Christmas Island in the
Indian Ocean (Daehler and Gordon 1997).
Another reason that not all introductions are necessarily detrimental is because
many introductions fail (e.g. the introduction of Puerto Rican Coqui Eleutherodactylus
coqui into New Orleans – see Dundee [1991]). In 1986, Williamson proposed his “10%
rule” which states that only 10% of the species that are introduced will establish self-
sustaining populations. This idea was later elaborated upon and became the “10-10 rule”
where 10% of introduced species become naturalized and 10% of the naturalized species
become pests (Williamson 1993). Williamson (1993) acknowledges that there is
considerable variation in this rule and that the rate for species becoming naturalized and
the rate for species becoming pests may vary from 5-20%. Nonetheless, his rule
illustrates that the vast majority of introductions fail to result in the naturalization of a
species. However, it has been proposed that organisms that are pests in part of their
22

native range are likely to become pests when they are introduced to other areas (Daehler
and Gordon 1997).
However, when a species does become naturalized and invasive, it may have
detrimental effects on the ecology and economy of a region. The negative effects of
introduced species can be broken down into the following six categories:
1.) Competition. Naturalized species may compete with native species for various
resources. House Sparrows Passer domesticus and European Starlings both compete with
native cavity-nesting species in the US (Lowther and Cink 1992, Cabe 1993). Similarly,
the introduced Common Myna Acridotheres tristis has been shown to outcompete native
parrots for nest cavities in Australia (Pell and Tidemann 1997).
In contrast, the Kaka Nestor meridionalis of New Zealand is forced to deal with
competition for food resources. During the summer, Kakas feed heavily on the
“honeydew” secreted by the scale insect Ultracoelostoma assimile. The introduced wasp
Vespula sp. also feeds on this “honeydew” and by the autumn the wasp is so numerous
that the Kakas are forced to switch to an alternative (and less energy-rich) food source. It
is believed that this competition for food is one of the major reasons for the decline of
Kakas in New Zealand (Beggs and Wilson 1991).
2.) Predation. Naturalized animals are sometimes amazingly successful predators in
their new location. The introduced Brown Tree Snake Boiga irregularis was so
successful as a predator on Guam that it drove several of the native forest birds into
extinction (Savidge 1987). Usher et al. (1988) also cites the example of a seabird colony
23

eing driven into extinction by rat predation. Lannoo et al. (1994) cite predation by
introduced Bullfrogs and Common Carp Cyprinus carpio as prime causes for the decline
of Blanchard’s Cricket Frog Acris crepitans blanchardi.
However, predation by an introduced animal will not always drive indigenous
species to extinction. When Mink were first introduced into southern Sweden, they
predated Common Eider Somateria mollissima nests heavily on islands where they had
been introduced, causing the Eiders to abandon these islands and to nest only on Mink-
free islands. However, Eiders have now begun to nest again on islands infested with
Mink, and it appears that the two species are able to co-exist as long as Mink numbers do
not increase (Usher 1988).
3.) Habitat alteration. Introduced and naturalized animals can also alter the habitat
itself. Muskrats and Coypu, for example, damaged swamps in Norfolk through excessive
herbivory (Baker 1990, Manchester and Bullock 2000). Feral pigs increased the rate of
nutrient mineralization as well as decreased the rate of nitrogen retention as they grubbed
for roots in the soil (Mack and D’Antonio 1998). Naturalized rhododendrons in Ireland
affect the regeneration of the forest by reducing the amount of light that reaches the forest
floor (Usher 1986). Introduced plants can cause changes in fire regimes, hydrology, soil
nutrient content, and geomorphology (Daehler and Gordon 1997, Mack and D’Antonio
1998).
4.) Disease. Introduced organisms can have a negative impact on native species by
importing disease. For example, it is believed that the introduction of mosquitoes onto
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Hawaii resulted in the introduction of avian pox and avian malaria, both of which
severely reduced the population of native Hawaiian birds (who were not resistant to these
diseases). It has been estimated that two-thirds of the species that were present in 1893
have been eliminated, primarily by these diseases (Pratt 1994). Similarly, it has been
speculated that the decline of Red Squirrels Sciurus vulgaris in Britain may be due in part
to diseases spread by the introduced Grey Squirrel Sciurus carolinensis (Baker 1990).
Zoonoses (diseases that spread from animals to people) may also be introduced by
introduced species, sometimes with disastrous results. It is believed that the introduced
Black Rat Rattus rattus, acted as a vector for the “Black Death” (bubonic plague) that
reached Britain in 1347 (Baker 1990). The death toll from this zoonotic disease was
sufficiently severe that 200 years elapsed before the human population returned to its pre-
plague level (Baker 1990).
5.) Hybridization. Although this is a less commonly demonstrated effect,
hybridization between native organisms and naturalized organisms does occasionally
occur. In Arizona, for instance, larval Tiger Salamander Ambystoma tigrum were
imported from other parts of the southwestern US and sold as bait. When these
introduced individuals escaped, they began hybridizing with the native Arizona
subspecies (Bury and Luckenbach 1976). In the UK, the introduced Sika Deer Cervus
nippon hybridizes with the native Red Deer Cervus elaphus (Baker 1990). North
American Ruddy Ducks Oxyura jamaicensis accidentally introduced into Britain have
now spread into Spain where they are hybridizing with the rare White-headed Duck
Oxyura leucocephala (Manchester and Bullock 2000).
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6.) Economic damage. In addition to ecological damage, naturalized species may also
cause economic damage. Naturalized Canada Geese Branta canadensis, often feed on
crops in the UK, causing substantial economic damage to the farming community (Owen
1990). Similarly, European Starlings introduced into the US can cause substantial
economic damage as they feed on planted crops, fruit trees, and food for cattle and other
livestock (Cabe 1993). Brown Tree Snakes introduced into Guam cause frequent power
failures which have been estimated to cost the local economy $4,500,000 per year (Fritts
2002). It has been estimated that the naturalized Rabbit Oryctolagus cuniculus population
in the UK cost £90,000,000-120,000,000 per year in lost revenue (Baker 1990).
Given the amount of ecological and economic damage that exotic species can
cause, a great deal of effort has been expended to try to predict if an introduced species
will become invasive. Such efforts have been largely futile, with most authors reporting
only limited success (e.g. Lawton and Brown 1986, Mollison 1986, Williamson and
Brown 1986, Gilpin 1990, Case 1996, Goodwin et al. 1999, Lockwood 1999, Blackburn
and Duncan 2001, Cassey 2002, etc.; but see Duncan et al. 2001). One of the major
problems with trying to predict the success of invaders is that there is an inherent bias
towards the recording of successful species – introduced species who are successful in
naturalizing are far more likely to be reported than those that failed to establish self-
sustaining populations (Usher 1988).
Nonetheless, many authors have reported some success in generating post hoc
theories as to why particular species have become established. Some authors believe that
26

there are certain life history and morphological characteristics that allow some species to
be introduced and naturalized successfully, while others believe that there are
characteristics of particular habitats that allow them to be invaded more successfully than
others.
A review of the literature reveals fourteen life history and morphological
characteristics that may allow a species a better chance to become established. They are
as follows:
Characteristics of successful invasive species
1.) Number of individuals introduced and the number of attempts made. The greater
the number of individuals introduced and the greater the number of introduction attempts
made, the greater are the chances that a species will have of becoming naturalized
(Grobler 1979, Williamson 1993, Veltman et al. 1996, Williamson and Fitter 1996,
Duncan et al. 2001, Duncan and Blackburn 2002).
Even species that have the potential to become pests may require multiple
introductions at multiple locations before they are able to establish a self-maintaining
population. European Starlings, for instance, were introduced at least a dozen times into
North America, but only one of these introductions was ultimately successful (Cabe
1993).
However, it is worth noting that even the release of a single pair of organisms can
occasionally result in the establishment of a self-sustaining population. Baker (1990) cites
the release of the Himalayan Porcupine in the UK as an example. A single pair escaped in
Devon in 1969 and began breeding successfully. These porcupines were observed
27

anging over 280 km 2 during the 1970s and the population was deemed to have become
naturalized (Baker 1990). They were eventually eliminated by ADAS (Agricultural
Development and Advisory Service).
2.) Greater reproductive potential. Species that are able to produce a large number of
young per individual are more likely to become naturalized than species that produce a
small number of young. For instance, highly fecund insects were found to be more likely
to establish naturalized populations (Crawley 1986, Lawton and Brown 1986). Similarly,
plants with longer flowering periods (Goodwin et al. 1999) were more likely to become
naturalized. Birds that lay more eggs per season (O’Connor 1986) and have higher
reproductive output (Veltman et al. 1996, Lockwood 1999) had a higher probability of
becoming established in non-native habitats.
3.) Widespread and abundant in its native range. The range size and abundance of an
organism have been linked to the probability of that species becoming established; the
greater the range size and the more abundant a species is, the more likely that an
introduced population will become naturalized (Williamson 1993, Goodwin et al. 1999,
Lockwood 1999, Duncan et al. 2001). Widespread and abundant species are more likely
to be exposed to a variety of conditions across their range and thus should be able to
inhabit a variety of conditions in other locations (Duncan et al. 2001). In addition,
widespread and abundant species may be caught and transported in greater numbers,
increasing the probability of successful introduction (Williamson 1993, Goodwin et al.
1999).
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However, Cassey (2002) argues against the validity of this hypothesis. He argues
that habitat generalism, rather than range size is more important. “It is important to note
that habitat generalism was not strongly correlated with geographical range so
suggestions that introduction success could arise because generalist species are more
abundant and therefore easier to catch and introduce in large numbers (Blackburn &
Duncan 2001b; Duncan et al., 2001) are not supported” (Casey 2002).
4.) Tolerant of abiotic factors. Species that are able to tolerate a wide variety of
abiotic factors may be more likely to become naturalized (Crawley 1986, Usher 1986,
Williamson 1993, Lockwood 1999, Cassey 2002). For example, Moyle and Light (1996)
demonstrated that exotic fishes that were able to withstand extreme variations in the
hydrologic cycle were more likely to establish naturalized populations and invade new
regions. Other authors have pointed out that the variety of climates available to a species
may be important in determining whether it establishes a self-sustaining population
(Holdgate 1986, Duncan et al. 2001).
5.) Genetically variable. Lockwood (1999) proposed that genetically variable species
were more likely to be successfully naturalized than genetically constrained species.
Unfortunately, however, this idea has not been explored in great depth. When organisms
are introduced into a new location, the limited number of individuals introduced may
result in a genetic bottleneck in the resulting naturalized population (e.g. European
Starling, Cabe 1993), and so genetically variable organisms may be less likely to suffer
from a genetic bottleneck. However, many introduced organisms do not appear to suffer
29

at all from a genetic bottleneck (Gray 1986). It is possible however, that there is an
inherent sampling bias, as many organisms with a severe genetic bottleneck may not
become naturalized.
Interestingly, after a sufficiently lengthy period in a new location, microevolution
of characteristics that differ from the founding population may occur. Lee’s studies on
Brown Anoles in Florida demonstrate that both meristic (pertaining to the number of
parts, e.g. numbers of scales; Lee 1985) and morphological (Lee 1987) characteristics
now differ from the source populations in the Caribbean and that the naturalized Florida
population now constitutes a distinct phenic type (i.e. individuals are identical in most
respects, they occupy a uniform area, and are clearly distinguishable from other groups;
Lee 1992). Similarly, cultural evolution in Eurasian Tree Sparrow Passer montanus song
memes in North America has been demonstrated with respect to the founding population
in Germany (Lang and Barley 1997).
6.) Commensal with humans. It is widely accepted that species that associate with
humans are more likely to establish naturalized populations (Lockwood 1999, Cassey
2002). The proportion of introduced and naturalized species is greater in urban areas than
in rural areas (Rebele 1994). The habitats created by extensive anthropogenic
modification of the landscape (e.g. the creation of a city) are often considered extreme
and are believed to be more easily inhabited by non-native “tolerant” species (e.g. those
that can tolerate a wide variety of abiotic conditions) than by the native species whose
habitats were so extensively modified (Rebele 1994; for a brief discussion on abiotic
tolerance, see above).
30

In addition, the proximity of these species to humans (and consequently human
methods of transport) means that there is a greater chance of them being transported
inadvertently into new areas (e.g. Brown Anolis in Florida, Zebra Mussels in the Great
Lakes – see above for a discussion on inadvertent introduction). However, some authors
believe that while anthropogenic transport may increase the number of species
introduced, it does not necessarily increase the probability of any given species becoming
established (Lockwood 1999, Blackburn and Duncan 2001).
7.) Body size. In general, it is widely agreed that organisms with larger body size
(within taxa) are more likely to become successfully naturalized (Crawley 1986, Massot
et al. 1994, Goodwin et al. 1999, Lockwood 1999, Duncan et al 2001, Casey 2002).
However, one study found that insects with smaller body size were more likely to
become established than those with a larger body size (Lawton and Brown 1986).
8.) Behavioural plasticity. Bird species that exhibit behavioural plasticity in foraging
and/or breeding in their native range may be more likely to become naturalized (Sol and
Lefebvre 2000, Sol et al. 2002). Dietary generalism has been correlated with increased
probability of successful introduction (Lockwood 1999, Duncan et al. 2001, Cassey
2002), presumably because these species will exhibit the behavioural plasticity needed to
utilize new foods successfully in areas where they have been introduced. Sol and
Lefebvre (2000) also linked invasion success in birds introduced into New Zealand with
brain size, which has been linked to behavioural plasticity. Sol et al. (2002) also
demonstrated a link between successful naturalization and brain size.
31

9.) Nonmigratory. Species that migrate seldom persist after being introduced, as their
migratory routes typically lead them to an unsuitable non-breeding habitat. Consequently,
nonmigratory organisms are more likely to become successfully naturalized (O’Connor
1986, Veltman et al. 1996, Lockwood 1999, Cassey 2002). However, Duncan et al.
(2001) were not able to support this hypothesis, as whether or not a bird species was
nonmigratory did not correlate with their success in establishing a self-sustaining
population in Australia.
10.) Low demographic stochasticity. The risk of stochastic extinction in an introduced
species varies inversely with the number of individuals released or escaped (Duncan et al.
2001). Consequently, a non-native species is more likely to become successfully
naturalized if a large number of individuals are introduced at multiple locations (as
described above). In addition, species with low demographic stochasticity are less likely
to suffer a stochastic extinction for a given population size and so low demographic
stochasticity has been correlated with introduction success (Lawton & Brown 1986,
Mollison 1986, Duncan et al. 2001, Casey 2002).
11.) Morphological dispersion. Morphologically similar species may compete with
each other for resources; consequently introduction success should be highest for those
species and ecosystems where morphologically dissimilar species are introduced
(Moulton and Pimm 1983, Duncan and Blackburn 2002). However, species can still
32

ecome naturalized even when morphological overdispersion is present (Lockwood et al.
1993, Lockwood et al. 1996).
12.) Taxonomy. It has been suggested that taxonomy plays an important part in
determining whether an exotic species will establish a naturalized population after being
introduced (Lockwood 1999, Duncan et al. 2001, Casey 2002). As certain life-history
characteristics that statistically predispose a species towards becoming successful are
shared among genera (e.g. nonmigratory, high fecundity, etc.), certain families may be
more likely to become successfully established. Among birds, Blackburn and Duncan
(2001) found that Phasianidae, Passeridae, Psittacidae, Anatidae, and Columbidae were
significantly more likely to become established than other families.
13.) Sexual monochromatism. Sexual monochromatism (also known as sexual
monomorphism, where males and females do not differ in plumage) has been linked to
the enhanced likelihood of an introduced species establishing a naturalized population
(Cassey 2002). Species with dichromatic plumage (and hence species with greater female
choice) tend to suffer from reduced fecundity when the numbers of available males are
limited (Møller and Legendre 2001).
14.) Dispersal of juveniles. It has been suggested (although not explicitly tested) that
low natal philopatry may allow an introduced species to expand its range and hence
become naturalized (Cabe 1993).
33

However, some authors feel that the fourteen life history and morphological
characteristics outlined above do not provide the best way to predict whether an exotic
organism will be successfully naturalized. Instead, they focus on the characteristics of an
ecosystem that may permit it to be invaded by non-native organisms.
Characteristics of successfully invaded habitats
Many authors have pointed out that introduced species are more likely to become
naturalized in disturbed ecosystems (Crawley 1986, Usher 1986, Baltz and Moyle 1993,
Case 1996). However, this is not always the case – some invasive species are able to
invade native habitats (Williamson and Fitter 1996). In general though, areas that have
been extensively modified by humans (e.g. cities) are more likely to host naturalized
populations than unmodified areas (Smallwood 1994).
Some authors have suggested that area may play an important role in allowing
introduced species to become naturalized. Case (1996) pointed out that island area
correlates positively with the number of species that have become naturalized. However,
other authors have not been able to correlate area with the number of species that have
been introduced and have self-maintaining populations (Moulton and Pimm 1983,
Smallwood 1994).
Perhaps the most contentious issue has been that of “biotic resistance”. In theory,
native species should be better adapted to native habitats than non-native species.
Consequently, when the full complement of native species is present in an ecosystem,
exotic species should be unable to establish self-sustaining populations because they will
be outcompeted (or, in some instances, predated) by native species. Several studies have
34

suggested that biotic resistance may be important in limiting the establishment of
introduced organisms (Crawley 1986, Baltz and Moyle 1993, Smallwood 1994).
However, a study on Anolis lizards in the Caribbean suggested that biotic resistance may
only limit the spread of a species rather than its probability of naturalization (Losos et al.
1993).
Still other studies on biotic resistance have found contradictory results, even while
studying the same system. For instance, Baltz and Moyle (1993) demonstrated that biotic
resistance limited the establishment and spread of exotic fishes in California (USA). In
contrast, Moyle and Light (1996) demonstrated that abiotic factors were more important
than biotic resistance in exotic fish naturalization in California. When appropriate abiotic
parametres for an exotic species were met, introduced fish could become naturalized,
regardless of the biotic resistance of the native fishes.
Riccardi (2001) proposed that biotic diversity (and hence theoretical biotic
resistance) actually benefits some introduced species. He found that positive interactions
(both commensal and mutualistic) among introduced aquatic organisms in the Great
Lakes increased at a faster rate than negative interactions.
Obviously, there is still much to learn about how introduced species become
established and what effects they may have on the ecology and economy of a region,
particularly species which are considered pests in part of their natural range.
The Rose-ringed Parakeet
One naturalized bird in particular that deserves further study is the Rose-ringed
Parakeet Psittacula krameri which has become naturalized in many countries throughout
35

the world, and which is a major crop pest in its native range (Forshaw 1989, Juniper and
Parr 1998). Despite the fact that Rose-ringed Parakeets are widely introduced, very little
work has been done on their biology in countries where they have been naturalized.
The Rose-ringed Parakeet is native to sub-Saharan Africa and India. It is a
medium-sized (38-42 cm) slim, green bird whose tail accounts for more than half the
length (up to 25 cm). The band of rose or light red encircles the neck of the male.
Females lack the rose-coloured ring, having instead only an indistinct emerald ring
around their neck. Males also have a black bib underneath their bill that extends down to
their “ring”. Many also develop a blue sheen on the back of their head. Immature birds
are difficult to distinguish from females, and are probably not separable in the field. In
general, the neck ring tends to be indistinct, as it is in females, but juveniles tend to
possess brownish fringes of the primaries and secondaries (Forshaw 1989, Juniper and
Parr 1998). Young males may begin developing their ring by the time they are three years
old (Forshaw 1989, Juniper and Parr 1998).
Four subspecies are currently recognized (Forshaw 1989, Juniper and Parr 1998):
1.) P. k. krameri: Found from Senegal east to west Uganda and southern Sudan
2.) P. k. parvirostris: Found from eastern Sudan east to northern Ethiopia and
Somalia.
3.) P. k. borealis: Found from Pakistan east through northern India (north of 20° N)
and through Nepal to Myanmar (Burma).
4.) P. k. manillensis: Found in India (south of 20° N) and Sri Lanka.
36

Figures 1 and 2 map the ranges of each subspecies.
The Rose-ringed Parakeet is the most widely introduced parrot in the world, with
populations apparently established in 35 countries on five continents (only Australia and
Antarctica remain uncolonized). They were first reported breeding in the UK in 1855 in
Norfolk but this colony soon disappeared (Lever 1977). During the 1930s, parakeets were
present in Epping Forest in Essex but this colony did not persist (Lever 1977, Morgan
1993). It was not until 1969 that a family group of parakeets was noted, this time in
Southfleet, Kent (Hudson 1974a, Lever 1977). By 1971, Rose-ringed Parakeets began
nesting near Croydon as well as at Esher (Lever 1977). Parakeets were also found at
Woodford Green-Highams Park area in southwestern Essex from 1971 onwards (Hudson
1974b). During 1972-1973 birds began breeding in Margate (Lever 1977). Starting in
1972, parakeets were observed near Old Windsor and Wraysbury (Lever 1977). By 1976
parakeets bred in Stockport in the Greater Manchester area (Lever 1977). By the late
1980s, a small population (< 12 birds) was also present at Studland, Dorset (Morrison
1997).
By 1983, the BOU accepted the Rose-ringed Parakeet as a Category C
(established exotic) species and estimated that the population consisted of 500 birds
(British Ornithologists' Union 1983). The birds appear to be a mixture of P. k. borealis
and P. k. manillensis (Morgan 1993). By 1986, the population was estimated to consist of
500-1000 birds (Lack 1986). A simultaneous count of the known roosts in 1996 revealed
that the population had increased to 1508 individuals (Pithon and Dytham 1999). Since
that time however, there has been a dramatic increase in the numbers of Rose-ringed
37

Parakeets. By 1999, there were approximately 2500 parakeets at one roost alone (Butler
2002).
So what detrimental effects might this species have? As outlined above,
naturalized species can have detrimental effects through competition, predation, habitat
alteration, disease, hybridization, and economic damage. It is unlikely that Rose-ringed
Parakeets will predate native animal species, as they are not predators. Similarly, it is
unlikely that Rose-ringed Parakeets will alter any habitats in the UK, as humans have
already heavily modified the areas they inhabit. It is a possible vector for Newcastle
disease (which could have a detrimental effect on the poultry industry in the UK), but to
date no instances of Newcastle disease have been attributed to this species. Finally, it is
unlikely to cause any damage through hybridization, as there are no closely related native
species in the UK with which it might hybridize.
Nonetheless, the continuing increase and spread of this species may be a cause
for concern for two reasons. First, it has been suggested that parakeets may have a
detrimental effect on other cavity-nesters as they begin nesting in late February or early
March, much sooner than most native species (England 1974, Tozer 1974, Lever 1977).
As nest cavities may be a limiting resource (Elliot et al. 1996, Mawson and Long 1994),
increasing numbers of Rose-ringed Parakeets could potentially claim greater numbers of
nest cavities causing populations of Kestrels Falco tinnunculus, Stock Doves Columba
oenus, Jackdaws Corvus monedula, European Starlings Sturna vulgaris, and other
secondary cavity nesters to decrease.
Secondly, Rose-ringed Parakeets are a major crop pest in India, attacking a
variety of grain products and fruit (Lever 1987, Long 1981, Mukherjee et al. 2000, Reddy
38

1998a, Reddy 1998b). Some authors consider these parakeets to be the biggest avian farm
pest in India (e.g. Shivanaray 1981, Mukherjee et al. 2000). For example, one study
found that Rose-ringed Parakeets in India reduced maize crop yields by up to 81%
(Reddy 1998a). Another study found that depredations by Rose-ringed Parakeets on
sorghum crops reduced yields by up to 74% (Reddy 1999b). The crop contents of 40
nestlings collected in India consisted primarily of pulses (such as black gram), cereals
(such as sorghum and maize) and oil seeds (such as sunflower; Shivanarayan 1981), a
result consistent with that obtained from crop contents of adult birds (Saini et al 1994).
As their population increases in the UK, it raises the possibility that they may
eventually become a crop pest in this country as well. Given the potential economic
impact of this species, it is surprising that the British government has not yet taken a
stance on parakeets.
The focus of this thesis will be to investigate the basic biology of Rose-ringed
Parakeets in the UK and will attempt to answer some basic questions such as: How many
parakeets are breeding in the UK and how rapidly is their population increasing? What
factors, if any, will limit their spread? How have populations in the UK adapted to local
conditions? Will this species eventually become either an ecological or economic pest in
the UK? And if they do eventually become a pest, what should be done?
39

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Williamson, M. H. and K. C. Brown. 1986. The analysis and modeling of British
invasions. Philosophical Transactions of the Royal Society of London, Series B,
Biological Sciences 314: 505-521.
Williamson, M. and A. Fitter. 1996. The varying success of invaders. Ecology 77: 1661-
1666.
51

FIGURES:
Figure 1: The range of the two African subspecies of Rose-ringed Parakeet, P. k. krameri
and P. k. parvirostris.
52

Figure 2: The range of the two Asian subspecies of Rose-ringed Parakeet, P. k. borealis
and P. k. manillensis.
53

Chapter 2
Population estimates and roosting behaviour
ABSTRACT:
The Rose-ringed Parakeet Psittacula krameri is an established exotic species in
southern England that gathers in communal roosts throughout the year. The reason for
this roosting behaviour is unknown, although a number of hypotheses have been
advanced for communal roosting in other species of birds. From a practical standpoint,
counts of communal roosts provide an estimate of the population size. It has also been
suggested that roost counts may be used to estimate the number of breeding pairs of
Rose-ringed Parakeets. Keiji (2001) suggested that the decrease in numbers of parakeets
at a roost during spring is due to females remaining on the nest while males return to the
roost. My objectives in this chapter are twofold: (1) to determine whether the population
of Rose-ringed Parakeets has increased (based on roost counts); and (2) to determine
whether adult male Rose-ringed Parakeets continue to utilize communal roosts during the
breeding season. Roost counts were conducted during winter and spring from late 2000 to
early 2002 and it was found that the population had increased to approximately 5800
individuals by early 2002. During the spring of 2002, 14 adult male parakeets were fitted
with tail-mounted radio transmitters. Although a high rate of transmitter failure was
experienced (with 35.7% of the transmitters lasting for less than a day), the birds were
indeed found to return to the roosts at night.
54

INTRODUCTION:
Rose-ringed Parakeets Psittacula krameri are native to the Indian subcontinent
and sub-Saharan Africa and are now an established exotic in the UK (Forshaw 1989,
Juniper and Parr 1998). However, multiple releases occurred before this species began
breeding regularly (see Chapter 1 for details). Since 1969, Rose-ringed Parakeets have
bred annually in the UK. During the winter of 1995/1996, the first simultaneous surveys
of the roosts were conducted, resulting in an estimate of 1,500 parakeets in the UK, and
the authors of this study suggest that parakeet numbers are only slowly increasing
(Pithon and Dytham 1999a). However, county bird reports for London, Surrey, and Kent
suggest that the population of parakeets has increased considerably since then (see Table
1).
Although the population has increased exponentially since the mid-1990s, little
work on the basic biology of this species in the UK has been published (but see Feare
1996, Pithon and Dytham 1999a, Pithon and Dytham 1999b). Indeed, even estimates of
the number of breeding individuals are lacking. It has previously been suggested that the
decrease in roost size from winter to spring in the Netherlands is due wholly to females
remaining on their nest while adult males continue to roost communally (Keiji 2001).
Observations during the breeding season in the UK confirm this. On 47 occasions when
nests were observed through sunset, it was found that shortly before sunset, all the male
parakeets leave their nests and fly in the general direction of the nearest roost (pers. obs).
Since spring roosts contain numerous adult males (pers. obs.), it is logical to suppose that
the males seen leaving nests in the evening turn up later at the roost. Females however,
have not been observed to leave their nest at night during the breeding season.
55

Consequently, the difference in roost sizes between the winter and the spring can be used
to estimate the number of breeding pairs. However, to date, the assertion that males leave
their nests to roost during the breeding season has not been verified.
Population monitoring
A number of techniques have been used to estimate the numbers of parrots during
the non-breeding season. They include:
1.) Mist netting
Mist nets are frequently employed in the Neotropics in order to study the local
avifauna (Whitman et al. 1997). Mist netting may increase detection rates of
inconspicuous, nonvocal species, provided that they are active within 2 m of the ground
(Whitman et al. 1997). As many parrot species are most active in the canopy (Gilardi and
Munn 1998), mist nets set on the ground may be less effective than those set in the
canopy (Meyers 1994). In addition the use of decoys and/or tape recordings of
conspecific calls may increase the numbers of parakeets trapped (Hussein et al. 1992,
Meyers 1994). However, while mist netting may reveal the presence of certain species of
psittaciformes, it is ill suited for estimating the numbers of parrots or parakeets (unless a
form of mark-recapture is employed – see method 5 below).
2.) Point counting
Point counting is a technique whereby an observer counts the numbers of birds
seen/heard at a series of points (Bibby et al. 2000). It is frequently used to estimate the
56

numbers of birds in an area (Casagrande and Beissinger 1997, Whitman et al. 1997). If a
fixed-radius point count is utilized (i.e. all the birds within a certain distance are counted
within a set time) it may be possible to extrapolate the numbers of parrots or parakeets
within a given area (Reynolds et al. 1980, Casagrande and Beissinger 1997, Marsden
1999). Suitable parametres need to be established before this technique can be used, such
as species detection curves and the appropriate length of time to count birds at a point
(Marsden 1999). Whitman et al. (1997) found that it detected a greater number of species
than mist netting. However, Casagrande and Beissinger (1997) found that while point
counting produced similar population estimates of Green-rumped Parrotlets Forpus
passerinus when compared to line transects, mark-recapture methods, and roost counts,
point counts tended to underestimate numbers in open habitats.
3.) Line Transects
Line transects simply involve walking a straight line and counting the numbers of
birds seen or heard from the line (Bibby et al. 2000). It differs from point counting in that
birds are counted continuously rather than at discrete points (Bibby et al. 2000). This
technique is frequently employed to count psittacines (e.g. Guix et al. 1999) and variants
are also sometimes employed, such as counting macaws while travelling in a motorized
canoe down a river (Kenton 2002). Casagrande and Beissinger (1997) concluded that
“line transect surveys more accurately estimated the distribution of the population
between habitats” and recommended that line transects be used rather than point counts
to estimate the numbers of parrots.
57

4.) Plot searches
Plot searches are where an observer searches a suitable habitat for a species, and for
species that hold discrete territories, territory mapping may be employed (Bibby et al.
2000). However, parrots and parakeets tend not to hold discrete territories (Forshaw
1989, Juniper and Parr 1998) and this technique is only infrequently used. Rodriguez-
Estrella et al. (1992) used this technique to census Isla Socorro (Mexico) for the endemic
subspecies of Green Parakeet Aratinga holochlora brevipes, and it may be suitable for
censusing psittaciformes on other small islands.
5.) Mark-recapture
Mark-recapture techniques are frequently employed to estimate population size and
life history parametres. With this method, individuals are captured and marked with an
individually distinctive mark during a session. The proportion of individuals caught
during the next session can then be used to estimate the population size (Bibby et al.
2000). The classic example of this is the Lincoln index that is given by the formula
an
P =
r
Where ‘P’ is the population size, ‘a’ is the number marked, ‘n’ is the number caught
during the second attempt, and ‘r’ is the number recaptured (Lincoln 1930). It is possible
to devise more sophisticated mark-recapture models than the two-sample Lincoln index
as well. For a review of these other models see Schwarz and Seber (1999) and Bibby et
al. (2000).
58

Bibby et al. (2000) present a summary of assumptions inherent in mark-recapture
models. These assumptions include:
A.) The population is closed or immigration and emigration can be measured or
calculated
B.) There is equal probability of capture in the first capture event
C.) The marked birds should not be affected by being marked
D.) The population should be sampled randomly in subsequent capture events
E.) All marked individuals occurring in the second or subsequent samples are
reported
F.) Capture probabilities are assumed constant for all periods
G.) Every bird in the population has the same probability of being caught in
sample i
H.) Every marked bird in the population has the same probability of surviving
from sampling periods i to i+1.
I.) Every bird caught in sample I has the same probability of being returned to
the population
J.) All samples are taken instantaneously such that sampling time is negligible
K.) Losses to the population from emigration and death are permanent
L.) Population closed to recruitment only
Mark-recapture studies have not often been performed with psittaciformes,
presumably because of the difficulty in capturing (or recapturing) them. Indeed,
recaptures of Rose-ringed Parakeets during the course of this study at sites where they
59

were ringed were quite low, with only 14 out of 264 birds (5%) recaptured during a two-
year period at one site in Surrey (see CH 6).
However, this technique has been used with great success to estimate life history
parametres of the Green-rumped Parrotlet (Sandercock and Beissinger 2002). A
comparison of four different methods of estimating parrot populations (point counts, line
transects, mark-recapture and roost counts) concluded that all four techniques provided
similar estimates of population size but that mark-recapture techniques had the greatest
(i.e. poorest) confidence intervals (Beissinger and Casagrande 1997).
6.) Roost counts
Many parrot species roost together in large flocks (Forshaw 1989, Juniper and Parr
1998). Roost counting is probably the most frequently used technique to estimate
psittaciform population size (Chapman et al. 1989, Gnam and Burchsted 1991, Mabb
1997, Pithon and Dytham 1999, Keiji 2001, Renton 2002). However, in order to be used
properly, it is important that the following three assumptions are met: 1.) That all parrots
are at the roost(s) when surveyed; (2) that all roosts have been located; and (3) the count
is accurate. Consequently, it may be easiest to carry out roost counts on small islands
with smaller numbers of birds (Beissinger and Casagrande 1997).
Most of these methods are potentially suitable for estimating the population of
Rose-ringed Parakeets. Mist nets are an exception because while they could be used to
determine whether the birds are present in an area, it is difficult to extrapolate population
estimates using this technique. Although the other methods described above may be
60

suitable, it should be noted that studies on psittaciformes have focused on native species.
Rose-ringed Parakeets, in contrast, are a recent addition to the British avifauna.
Consequently, their distribution may still be patchy, with apparently suitable habitat
unoccupied. This means that it would be difficult to extrapolate from the density
estimates provided by several of the techniques outlined above (e.g. point counts, line
transects, plot searches, and mark-recapture studies) to a total population estimate.
Consequently, it appears that the best way to estimate the number of Rose-ringed
Parakeets in the UK is by counting at roosts. This is the same technique that was used by
Pithon and Dytham (1999) in a previous study on Rose-ringed Parakeets in the UK (who
located roosts through appeals to the public for information on roosts). However, it
should be noted that these population estimates could only be based on known roosts, and
so represent a minimum estimate of the population.
Estimating the number of breeding pairs
Although the population has increased exponentially since the mid-1990s, little
work on the basic biology of this species in the UK has been carried out. Indeed, even
estimates of the number of breeding individuals are lacking. It has previously been
suggested that the decrease in roost size from winter to spring in the Netherlands is due
wholly to females remaining on their nest since adult males continue to roost
communally (Keiji 2001). Observations during the breeding season in the UK confirm
this (pers. obs.). Shortly before the sun sets, all the male parakeets leave their nests and
fly in the general direction of the nearest roost. Since spring roosts contain numerous
adult males, it is logical to suppose that the males seen leaving nests in the evening turn
61

up later at the roost. Females however, have not been observed to leave their nest at night
during the breeding season. Consequently, the difference in roost sizes between the
winter and the spring can be used to estimate the number of breeding pairs. However, to
date the crucial assumption necessary to estimate the number of breeding pairs (i.e. that
the males return to the roost) has not been tested in the literature. Ideally, breeding adult
male birds should be marked in such a manner that they could easily be detected at the
roost.
There are a number of different methods for marking individual birds. However,
many of these techniques may not be suitable for Rose-ringed Parakeets. For instance,
patagial tags have been used to identify individual birds (Smith and Rowley 1995, Carver
et al. 1999, Bibby et al. 2000). However, some studies have found evidence that patagial
tags modify behaviour and suggested that they not be used (Brua 1998). Bibby et al.
(2000) suggest that patagial tags may increase mortality. Plumage dyes have also been
used to mark birds (Wendeln et al. 1996, Bertollotti et al. 2001) including Monk
Parakeets in Argentina (Eberhard pers. comm..). However, an attempt to study Rose-
ringed Parakeet movements by using black hair dye to dye a number on their breast was
unsuccessful as the dye faded in three weeks and was seldom visible unless excellent
views were obtained (Butler, unpubl. data). Coloured rings are often used to mark birds
(e.g. Beletsky and Orians 1989). However, the tarsus of a Rose-ringed Parakeet is too
small to fit more than one ring per leg, which limits the usefulness of this technique (pers.
obs.). In contrast, radiotelemetry has been used to successfully track parrots in the field
(e.g. Meyers et al. 1996, Sanz and Grajal 1998) and this technique should be suitable for
tracking Rose-ringed Parakeets.
62

The object of this chapter is to test the following two hypotheses. (1) The
population of Rose-ringed Parakeets in the UK has increased substantially since 1996; (2)
Adult males return to the roost during the breeding season.
METHODS:
Roost counts
Requests for roosting locations were made through the media (newspaper,
television, radio, and internet) and were then independently verified. In addition, roosts
were actively searched for during the evening. Five roosts were counted during the winter
(November to February) and spring (March to May) of 2000/2001 and 2001/2002 by the
author and volunteers. A single observer (Dr. Grant Hazlehurst) collected all the roost
count data at Lewisham, while two other volunteers each provided a single roost count
for Ramsgate and Reigate. All other roost count data were collected by the author.
Surveys were conducted from locations that allowed a full view of parakeets entering or
leaving the roost. Although Pithon and Dytham (1999a) speculated that parakeets may
travel between roosts, no evidence that individual Rose-ringed Parakeets move from one
roost to another has been presented in the literature. Consequently, roost counts were not
conducted simultaneously.
Two methods were used to collect roost count data, depending upon the size of
the roost. First, birds were counted after they had settled into their roosting trees for the
evening. However, as parakeets frequently do not arrive until after dusk, this method is
only practical for small roosts (i.e. those roosts of less than 600 birds) as the rapid onset
63

of darkness makes it impossible to count large numbers of parakeets. Furthermore, as tree
branches often obscured parakeets, numbers were likely to be underestimated. The
second method was to count flying parakeets as they arrived at, or left the roost. When
possible, birds were counted individually but if flock sizes were large (e.g. 50+ birds),
parakeets were counted in groups of five (Bibby et al. 2000). This method was employed
on those roosts of more than 600 birds. Evening roost counts began 40 minutes prior to
sunset and continued until 30 minutes after sunset. Dawn roost counts began 30 minutes
prior to sunset and continued until the last bird had left the roost.
The four roost locations described by Pithon and Dytham (1999a) were visited.
They include Esher Rugby Football Club in Hersham, Surrey (51º 22’ 52” N x 0º 23’ 24”
W); the Reigate/Redhill area of Surrey (51º 14’ 29” N x 0º 10’ 37” W); Lewisham
Crematorium in Lewisham, Kent (51º 26’ 10” x 0º 0’ 40” E); and the Ramsgate Rail
Station in Ramsgate, Kent (51º 20’ 29” N x 1º 24’ 25” E). In addition, a new roost was
discovered at the Bray Gravel Pits near Maidenhead, Berkshire (51º 30’ 10” N x 0º 41’
26” W). A map of these locations can be seen in Figure 1.
At the Lewisham Crematorium location, roost counts were carried out throughout
the year. In addition, flock sizes of arriving birds were noted, as well as the direction
from which they arrived. These detailed roost counts were only possible because a
volunteer lived near the roost and was willing to record this information on a regular
basis. Distance and financial constraints limited the opportunity for recording this
information at the other known roosts.
64

Estimating the number of breeding pairs
In total, 14 adult male parakeets were followed using radiotelemetry during the
spring of 2002. (Females were not marked with radio transmitters as they have not been
observed leaving the nest during the breeding season.) The parakeets were caught in mist-
nets while coming to bird feeders. A previous study on attaching transmitters to this
species demonstrated that Rose-ringed Parakeets were capable of removing transmitters
attached to the leg and reacted poorly to a transmitter mounted on a harness (Pithon
1998). Consequently, the transmitters were mounted (i.e. sewn) on the tail, a procedure
suitable for species with large, powerful bills (Bray and Corner 1972, Kenward 1978) and
which has been successfully used on parrots (Smales et al. 2000). The transmitters,
purchased from BioTrack (Model TW-4), weighed 2.4 g with a range of approximately 1
km and had a battery life of six weeks. Radio tracking was done with a three-stage Yagi
antenna and was carried out three times a week (while searching for nests) from March
through May 2002.
Statistics were calculated with SPSS v.11. Results are presented as mean ±
standard deviation, except where noted.
RESULTS:
Population monitoring
On the basis of roost counts, the estimated population size of Rose-ringed
Parakeets in the UK has increased substantially since Pithon and Dytham’s (1999a) 1996
study. During the winter of 2000/2001, approximately 4200 parakeets were located at the
four major roosts (Esher, Reigate, Lewisham, and Ramsgate; see Table 2). This
65

epresents a 280% increase over the number of parakeets reported by Pithon and Dytham
(1999a) in 1996.
The number of birds roosting communally decreased from winter to spring.
During the spring of 2001, the roost counts at Esher, Lewisham and Ramsgate (the roost
at Reigate could not be relocated) showed a population of approximately 3000 parakeets.
By the winter of 2001/2002 the number of parakeets at these four roosts had increased by
35% (from the winter of 2000/2001) to approximately 5700 birds. In addition, a new
roost was discovered in Maidenhead, bringing the total to approximately 5800 (see Table
2). By the spring of 2002, there were approximately 4000 parakeets at the three largest
roosts (Esher, Lewisham and Ramsgate; the roosts at Reigate and Maidenhead could not
be relocated during this period; see Table 2).
It is worthwhile to consider whether the difference in roost counts from winter to
spring may simply be due to count variation. The number of counts conducted at each
roost during the spring and the winter was fairly low and so statistical tests cannot easily
be employed. However, a total of 12 counts were made between November 2000 to April
2001 and 14 counts from November 2001 to April 2002 at Lewisham (for a total of 26
counts), an adequate sample size for statistical tests. The difference between the winter
2000/01 roost count (580 ± 73) and the spring roost count (435 ± 58) was statistically
significant (one-way ANOVA, F1,10 = 14.915, p = 0.003). Similarly, the difference
between the winter 2001/02 roost count (812 ± 71) and the spring 2002 roost count (560
± 91) was statistically significant (one-way ANOVA, F1,12 = 33.399, p < 0.001). Roost
counts during the winter of 2001/02 were significantly higher than roost counts during
the winter of 2000/01 (one-way ANOVA, F1,12 = 33.414, p < 0.001). Similarly, spring
66

oost counts during 2002 were significantly higher than spring roost counts during 2001
(one-way ANOVA, F1,10 = 8.547, p = 0.015). Since it is unlikely that the population
dynamics at the Lewisham roost differ from the population dynamics at the other roosts,
the seasonal and annual difference in roost counts at each of the other roosts probably
reflects actual changes in the number of parakeets roosting at each location, rather than
count error.
Estimating the number of breeding pairs
The tail-mounted transmitters experienced a high rate of failure, with 5 of the 14
(35.7%) transmitters failing by the second day (see Table 3). Thirteen of the 14 adult
males with radiotransmitters were located at the roost during the evenings. The 14 th bird
was never relocated and it is presumed that he destroyed the transmitter within a few
hours of its attachment.
Although the transmitters supposedly had a range of 1 km, they did not perform
this well in an urban environment. At best (e.g. when a bird flew by overhead) it was
possible to receive a signal from up to 200 m away. More often, however, it was
necessary to be within 25 m of the bird before a signal was detected.
Two males were located at their presumed nest cavities during April and May. It
was not possible to obtain permission to monitor these nests, as one was in a Royal Park
and the other was in a golf course. Consequently, few observations of these birds were
made – each bird was initially located in April, and then the nest sites were visited again
in May. Both birds were still present at their nest sites in May, although the radio
transmitters had ceased working by then.
67

The results from the tail-mounted transmitters confirm that these adult males
return to the roost during the breeding season, although the low sample size (n = 2)
means that these results are tentative at best. Assuming that all males return to the roost,
it appears therefore that the differences between winter and spring roosts are due to
females remaining on their nests. By this method, there were approximately 950 pairs
present in the UK during the spring of 2001 and approximately 1350 pairs present in the
UK during the spring of 2002 (see Table 2).
It is possible that there is some post-winter dispersal of non-breeding birds from
the roost. However, roosts are present year-round at Lewisham, Esher, and Ramsgate
(pers. obs). Year-round counts of parakeets at Lewisham Crematorium (see Figure 2)
indicate that only approximately 30% of the parakeets depart from the roost during the
spring, most of which are probably breeding birds.
DISCUSSION:
Population monitoring
Contrary to some recently published papers (Pithon and Dytham 1999a, Pithon
and Dytham 2002), the population of Rose-ringed Parakeets in SE London and the Isle of
Thanet, Kent is continuing to increase. Given the slow rate of increase from 1969-1996,
the dramatic recent increase in the Rose-ringed Parakeet population is startling (see Fig.
2). However, a similar trend has been noted in the Netherlands, where the population has
increased at a rate of 22.5% per year since 1994 (Keiji, 2001). Similarly, naturalized
populations of other introduced parrots often experience rapid population growth (e.g.
Enkerlin-Hoeflich and Hogan 1997, Brightsmith 1999).
68

The reason for the dramatic increase in numbers recently is unknown, although
two possibilities suggest themselves. The first is that warmer temperatures may be
affecting survival or reproduction. Warmer springs have been linked to increasingly early
laying dates in some birds in the UK (Crick et al. 1997, Crick and Sparks 1999).
Although these warmer temperatures may not cause the parakeets to be laying eggs
earlier, it is possible that the warmer springs may enable them to breed more successfully.
A second possibility is that estimates of the population sizes in the 1980s may
have been too high. Parakeets are both noisy and highly visible, and it is possible that this
may have resulted in the false impression that they were more numerous and widespread
than was actually the case. It is worth noting that the roost counts for this species had not
been undertaken when the BOU released its estimate of 500 birds in 1983 (BOU 1983). If
the population of breeding birds was lower than the BOU’s estimate, than it is possible
that it may have been increasing at a rate of 25-30% since the 1980s.
In addition, although some sources state that Rose-ringed Parakeets only roost
during the winter (e.g. Wernham et al. 2002), this was certainly not the case for the three
largest roosts (Esher, Lewisham and Ramsgate). As illustrated in Figure 2, parakeets
roosted year-round at Lewisham. In addition, roosts were consistently seen at Esher and
Ramsgate year-round (pers. obs.) although no counts were made of birds during summer
and fall. It is possible that the smaller roosts (e.g. Reigate and Maidenhead) were present
year-round too, but were not detected due to their small size and/or the difficulty of
locating green birds in actively photosynthesising vegetation.
69

Estimating the number of breeding pairs
Adult males with radiotransmitters were consistently observed returning to the
roost (see Table 4). This supports the hypothesis that the differences between winter and
spring roost sizes are due almost wholly to females remaining on their nests. However,
the limited range of the radiotransmitters (typically only 25 m), made locating parakeets
away from the roost very difficult. Only two males were located at their breeding sites.
Consequently, these results are tentative at best.
Parakeets are present at roosts year-round in Esher, Ramsgate (pers. obs.) and
Lewisham (see Figure 2). It is unlikely that the difference in roost numbers could be due
to non-breeding birds spontaneously leaving the roost during the spring as there is no
clear benefit to offset the increased vulnerability to potential predators (e.g. Peregrine
Falcon Falcon peregrinus and Sparrowhawk Accipiter nisus) and loss of information-
sharing that would result.
Obviously, some mortality may occur from the winter to the spring, which would
tend to inflate the number of breeding pairs. However, parrots in general are K-selected
and tend to have low annual mortality. Rose-ringed Parakeets generally live for 20 years
in captivity (Pithon 1998) and may live for as long as 34 years (Brouwer et al. 2000).
Although some mortality undoubtedly occurs, it is unlikely to have much of an effect on
estimates of the breeding population.
Although uncommon, reports of males of other species returning to a roost while
breeding are not unheard of. White Wagtails Motacilla alba, for instance, may join
communal roosts during the breeding season (Zahavi 1971). Similarly, adult Crested
Caracaras Caracara plancus, Brown-headed Cowbirds Molothrus ater, and American
70

Crows Corvus brachyrhynchos have all been observed using communal roosts during the
breeding season (Thompson 1994, Johnson and Gilardi 1996, Caccamise et al. 1997).
Pithon and Dytham (1999a) suggest that the parakeets roosting in the
Reigate/Redhill (Surrey) area may occasionally rejoin the flock at Esher, as they were
often unable to find this flock during their study. However, we were unable to verify this.
(Gnam and Burchsted [1991] also doubted that Bahama Parrots Amazona leucocephala
bahamensis moved between roosts.) Although this flock shifted seven times during 2000-
2002, it always remained in the Reigate/Redhill area.
It has been estimated that Rose-ringed Parakeets in the UK produced an average
of 0.8 young per nest, based on 12 nests observed during 1997-1998 (Pithon and Dytham
1999b). However, it appears that the rate of reproduction must be higher than this in
order to account for the 25% annual increase in the parakeet population since 1996. For
instance, it was estimated that there were 867 ± 199 pairs of parakeets at the Esher,
Lewisham, and Ramsgate roosts (out of a total population of 3932 ± 195) during the
winter of 2000/2001. These pairs should have produced 694 ± 159 new birds (based on
Pithon & Dytham's (1999b) estimate of 0.8 young per pair), bringing the population to
approximately 4600 (assuming no mortality) by the winter of 2001/2002. However, 5333
± 113 birds were actually recorded at these roosts. Therefore, unless significant
immigration took place, the pairs must have produced approximately 1400 new young, or
an average of approximately 1.6 young per nest. Given that some mortality certainly
occurred between when the young fledged and when they were counted at the roost
during the winter, it is likely that the actual fledging rate is somewhat higher. A recent
71

study on 108 nests of this species during 2001-2003 confirmed that fledging rates
averaged 1.9 ± 0.1 young per nest (See Chapter 4).
It should be noted that these calculations assume a closed population; that is, that
there were no introductions into the population. Although a few Rose-ringed Parakeets do
escape annually, it is very unlikely that the difference between the Pithon and Dytham’s
(1999a) estimate of population increase and the actual population increase is due to
escapees, as approximately 700 parakeets would have had to have been released during
2001 in order to account for the difference. It seems far more likely that Rose-ringed
Parakeets are more productive in the UK than previously thought.
Whether Rose-ringed Parakeets will expand their range yet further within the UK
remains to be seen. Currently, much of the population is located in suburban areas.
However, parakeets are now being seen in rural areas in Berkshire, Buckinghamshire,
and Surrey. If Rose-ringed Parakeets continue to increase at their current rate of 25-30%
per year, it may not be long before they spread into the British countryside (see Chapter
7).
72

Literature Cited:
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78

TABLES:
Table 1: A summary of roost high counts from 1995-1999. Data from 1996 are taken
from Pithon and Dytham (1999a). All other data are from the relevant county recorders
for the local bird clubs.
Year Esher Lewisham Reigate Ramsgate
1995 692 - - 130
1996 1123 85 118 299
1997 1507 275 123 314
1998 1704 210 75 437
1999 2500 - - 480
80

Table 2: Monthly roost counts from November 2000 – April 2002. A hyphen (-) indicates
that no counts were obtained for that month, either because the roost could not be
relocated or due to other logistical difficulties.
Location Nov
Dec
2000 2000
Jan
2001
Feb
2001
Mar
2001
Apr
Nov
2001 2001
Dec
2001
Jan
Feb
2002 2002
Mar
2002
Maidenhead - - - - - - 116 118 - - - -
Apr
2002
Esher 2672 2999 3080 - - 2327 - 3894 4096 - 3154 3078
Reigate - - - 277 - - 350 - - - - -
Lewisham - 625 626 550
± 86
(N =
3)
502 ±
66
(N =
2)
408
± 28
(N =
5)
867
± 45
(N =
3)
842 ±
58
(N =
2)
803
± 1
(n =
2)
708
± 6
(N =
2)
580 ±
120
(N =
3)
529 ±
37 (N
Ramsgate - 435 - - 293 205 - - 540 - - 317
Winter (Nov-Feb) Spring (Mar-Apr)
2000/2001
2001
Winter
2001/2002
= 2)
Spring (Mar-Apr)
Maidenhead - - 117 ± 1 -
2002
Esher 2917 ± 216 2327 3995 ± 143 3116 ± 54
Reigate 277 - 350 -
Lewisham 580 ± 73 435 ± 58 812 ± 71 560 ± 91
Ramsgate 435 249 ± 62 540 317
Total 4235 3011 5814 3993
81

Table 3: A summary of the radiotelemetry results. N = Number of days signal detected
Frequency
(in MHz)
Location
(City,
County)
173.352 Westgateon-Sea,
Kent
173.467 Westgateon-Sea,
Kent
173.343 Westgateon-Sea,
Kent
173.455 Thames
Ditton,
Surrey
173.323 Thames
Ditton,
Surrey
173.394 Hampton,
Middlesex
173.374 Betchworth,
Surrey
173.385 Bexley,
Kent
173.475 Bexley,
Kent
173.304 Hampton,
Middlesex
173.406 Hampton,
Middlesex
173.363 East
Molesey,
Surrey
173.293 East
Molesey,
Surrey
173.313 East
Molesey,
Surrey
Date
affixed
8 Mar
2002
8 Mar
2002
8 Mar
2002
22 Mar
2002
22 Mar
2002
23 Mar
2002
24 Mar
2002
2 Apr
2002
2 Apr
2002
4 Apr
2002
4 Apr
2002
22 Apr
2002
22 Apr
2002
22 Apr
2002
Roost where
detected
Ramsgate
Rail Station
Ramsgate
Rail Station
Ramsgate
Rail Station
Distance
travelled
Last date
detected
6.6 km 16 Mar
2002
6.6 km 8 Mar
2002
6.6 km 8 Mar
2002
Esher RFC 4.2 km 22 Apr
2002
Esher RFC 4.2 km 20 May
2002
Total
time
8 days
(n = 3)
< 1 day
(n = 1)
< 1 day
(n = 1)
31 days
(n = 4)
59 days
(n = 7)
Esher RFC 5.2 km 22 Apr 30 days
2002 (n = 4)
- - - < 1 day
(n = 1)
Lewisham 7.3 km 20 Apr 18 days
Crematorium
2002 (n = 3)
Lewisham 7.3 km 20 Apr 18 days
Crematorium
2002 (n = 3)
Esher RFC 5.9 km 20 May 46 days
2002 (n = 5)
Esher RFC 5.9 km 4 Apr < 1 day
2002 (n = 1)
Esher RFC 3.8 km 20 May 28 days
2002 (n = 4)
Esher RFC 3.8 km 22 Apr
2002
Esher RFC 3.8 km 20 May
2002
< 1 day
(n = 1)
28 days
(n = 4)
82

Table 4: A summary of radiotelemetry visits to the roosts. An “X” indicates that the roost
was visited on that day. The series EG5xxxx indicates the ring number of the bird
detected with radiotelemetry. Each column indicates the location where the bird was
detected (“Rams” is an abbreviation for Ramsgate, and “Lewis” is an abbreviation for
Lewisham)
8/3/02 9/3/02 16/3/02 23/3/02 2/4/02 3/4/02 4/4/02 20/4/02 22/4/02 20/5/02
Esher X X X X X
Lewisham X X
Ramsgate X X X
EG50375 Rams Rams Rams
EG50376 Rams
EG50378 Rams
EG50386 Esher Esher Esher Esher
EG50387 Esher Esher Esher Esher Esher
EG50389 Esher Esher Esher
EG50393 Lewis Lewis
EG50394 Lewis Lewis
EG50397 Esher Esher Esher
EG50399 Esher
EG51252 Esher Esher
EG51259 Esher
EG51260 Esher Esher
83

FIGURES:
Figure 1: The locations of the five major roosts.
84

Figure 2: Average (± stdev) roost counts at Lewisham Crematorium for the period
December 2000 to October 2002. No roost counts were conducted during October 2001
Number of parakeets counted
1600
1400
1200
1000
800
600
400
200
0
Dec-00
Jan-01
Feb-01
Mar-01
Apr-01
May-01
Jun-01
Jul-01
Aug-01
Sep-01
Oct-01
Nov-01
Dec-01
Jan-02
Feb-02
Mar-02
Apr-02
May-02
Jun-02
Jul-02
Aug-02
Sep-02
Oct-02
Date
Dec-00 Jan-01 Feb-01 Mar-01 Apr-01 May-01 Jun-01 Jul-01 Aug-01 Sep-01 Oct-01 Nov-01
Count N = 1 N = 1 N = 3 N = 2 N = 5 N = 2 N = 5 N = 3 N = 2 N = 3 N = 0 N = 3
Dec-01 Jan-02 Feb-02 Mar-02 Apr-02 May-02 Jun-02 Jul-02 Aug-02 Sep-02 Oct-02
Count N = 2 N = 2 N = 2 N = 3 N = 2 N = 3 N = 2 N = 3 N = 2 N = 1 N = 2
85

Chapter 3
Factors influencing Rose-ringed Parakeet naturalization probability
ABSTRACT:
Rose-ringed Parakeets Psittacula krameri are the most widely introduced parrot
species in the world, but to date no studies have been conducted to identify factors that
contribute to their success as an invasive exotic. In this chapter, I explore factors that
determined the realized population size as well as the probability of becoming naturalized
(i.e. the probability of establishing a self-sustaining population). Using the 17 intrinsic
and extrinsic factors hypothesized to affect a species probability of becoming naturalized
(as outlined in Chapter 1) five hypotheses were selected for a quantitative test. Data from
each country on introduction effort (using human population size as a surrogate),
morphological dispersion (e.g. the number of introduced and native parrot species),
elapsed time between first successful breeding and the latest population estimate, average
number of frosts per year, and area (of the country) were put into both a General Linear
Model (to predict realized population size) and a binary logistic regression (to predict
whether Rose-ringed Parakeets will become naturalized in a given country). A lack of
basic data on current population sizes hindered the analysis, as did a lack of information
on unsuccessful introduction attempts (only four are reported). It was not possible to
create a significant GLM to predict Rose-ringed Parakeet population sizes (p = 0.147, n =
18). Similarly, the binary logistic regression was not successful in separating countries
where Rose-ringed Parakeets did or did not become naturalized (p = 0.078, n = 38).
86

However, a significant discriminant function was created that correctly classified 92.1%
of cases (p = 0.025, n = 38). This discriminant function utilized the date of introduction,
the numbers of introduced parrot species, and the area.
87

INTRODUCTION:
The Rose-ringed Parakeet Psittacula krameri is the most widely introduced parrot
species in the world (Long 1981, Lever 1987, Forshaw 1989). The reasons for its success
as an introduced species have not been explored. A number of hypotheses have been
advanced to explain why certain species tend to be introduced successfully, while others
are not. In general, these theories can be grouped under two general headings: (1) a given
species is intrinsically predisposed to establishing a population successfully when
introduced into a new area because of its life history characteristics; and (2) habitat
characteristics determine whether a species will become naturalized in a given location.
However, many of these hypotheses are difficult to test with regards to the Rose-
ringed Parakeet. The number of individuals released and the number of release attempts
made can easily be tested when the data are available. For instance, when birds are
released in an organized fashion by groups (Cabe 1993, Lowther and Cink 1993) or by
government agencies (Veltman et al. 1996, Duncan and Blackburn 2002) records are
often kept on the numbers of birds released and the numbers of attempts made. In
contrast, the releases of parrots and parakeets that occurred were accidental in nature (e.g.
escaped pets, escaped shipments of birds, etc; see Forshaw 1989, Juniper and Parr 1998)
and records are seldom kept on accidental releases.
Greater reproductive potential is only a relevant life history characteristic when
studying more than one species of bird (Crawley 1986, Veltman et al. 1996), as is the
issue of whether a species is widespread and abundant (Duncan et al. 2001).
A species’ tolerance of abiotic factors is a relative factor (i.e. it depends upon
comparing more than one species; see Usher 1986, Williamson 1993), but it can easily be
88

tested on a single-species system. Duncan et al. (2002) achieved a great deal of success in
predicting the amount of territory an exotic species would occupy in Australia by
matching climatic factors from species’ native ranges with similar climates in Australia.
However, Rose-ringed Parakeets occupy a wide natural range, and climatic conditions in
their native habitats range from cool and wet (e.g. northern India in the Himalayas) to hot
and dry (e.g. the savannas of Africa; Juniper and Parr 1998). Such a broad tolerance for
climatic conditions suggests that climate matching would be of little relevance to this
species. There is some indication, however, that minimum temperatures may be an
important factor in determining whether introduced populations become naturalized.
Rose-ringed Parakeets introduced into New York City, NY (USA) suffered from frostbite
during the winter (Roscoe et al. 1976). Similarly, Rose-ringed Parakeets introduced into
Belgium have been known to suffer mortality due to winter weather (Temara and
Arnhem 1996). Consequently, an analysis of the factors that influence the probability of
survival should include minimum winter temperatures.
It has been suggested that species with greater genetic variation (than other
species) have a greater chance of becoming naturalized (Lockwood 1999). However, this
is an avenue of research that has not yet been explored in Rose-ringed Parakeets and so
cannot be included in the analysis. Another factor that cannot be easily incorporated into
this analysis is a species’ propensity for commensalism with humans. This is only
valuable when comparing several species (Cassey 2002) and is difficult to quantify in any
case. Similarly, whether a species’ body size influences its probability of naturalization
can only be ascertained by comparing one species with another (Massot et al. 1994).
Similarly, dietary generalism (Lockwood 1999), nonmigratory tendencies (O’Connor
89

1986), and low demographic stochasticity (Lawton and Brown 1986, Casey 2002) are all
relative factors, best employed in comparative studies of several species.
Morphological dispersion (Mouton and Pimmm 1983, Ducan and Blackburn
2002), on the other hand, is a factor that can easily be used on a single species. It has
been suggested that the probability of a species becoming established is higher if no other
morphologically similar species are present (Moulton and Pimm 1983, Duncan and
Blackburn 2002). Rose-ringed Parakeets have been introduced into locations where no
other parrot species is established (e.g. Great Britain; see Pithon and Dytham 1999a), and
into locations where native parrots were already present (e.g. Kenya; see Cunningham-
van Someren 1970) as well as locations where other exotic species occurred (e.g.
California; see Garrett 1997 and Mabb 1997). Indeed, Nebot (1999) has suggested that
Rose-ringed Parakeets in Venezuela are restricted to Caracas due to competition with
native species outside the city. An analysis of the success rate of introduction in each
scenario may determine whether the number of parrot species present affects the Rose-
ringed Parakeet’s probability of becoming naturalized.
Another avenue of research that might be worthwhile is to examine the number
and abundance of cavity-nesting species in countries where Rose-ringed Parakeets have
been introduced. However, there are several problems with this approach. First, there is
the obvious problem of ascertaining which species might be competing with Rose-ringed
Parakeets, as Rose-ringed Parakeets are frequently introduced into urban areas (see Table
1) where native cavity-nesting species may be scarce (e.g. Nebot 1999). Secondly, while
in some instances cavities may be a limiting factor for some species (e.g. Newton 1994),
studies have suggested that Rose-ringed Parakeets are not limited by cavities in countries
90

where they are introduced (Pithon and Dytham 1999b). Thirdly, these parakeets appear to
have little difficulty in defending nest cavities from other species (e.g. Sarwar et al. 1989,
pers. obs.). Provided that Rose-ringed Parakeets do not begin breeding after native
species (which is unlikely as they begin breeding in February – see Juniper and Parr
(1998)), their ability to defend cavities successfully means that they are unlikely to suffer
from competition with other native species. Finally, parrots as a group tend to undergo
exponential population increases when naturalized in a new country (e.g. Red-crowned
Parrot Amazona viridigenalis, Rose-ringed Parakeet, Yellow-chevroned Parakeet
Brotogeris versicolurus, White-winged Parakeet Brotogeris chiriri, Monk Parakeet
Myiopsitta monachus, etc.; see van Bael and Pruett-Jones 1996, Enkerlin-Hoeflich and
Hogan 1997, Spreyer and Bucher 1998, South and Pruett-Jones 2000, Butler 2002, Pranty
2002). This implies that competition with native cavity-nesting species does not limit the
numbers of parakeets.
The importance of taxonomy in the establishment of a species is only relevant
when considering a suite of species (Duncan et al. 2001). The link between sexual
monochromatism and the probability of a species becoming naturalized is interesting (see
Casey 2002), but again is only relevant if more than one species is considered. The
distance that juveniles disperse may affect a species’ chances of establishing a self-
sustaining population (Cabe 1993), but again this is a relative variable, best employed
when examining naturalization patterns across species. The degree of disturbance in an
ecosystem is worth considering (Case 1996), but little information is available about it in
the literature (although it should be noted that most introductions of Rose-ringed
Parakeets occur in urban areas).
91

Area has been correlated with the number of naturalized species (Case 1996).
Although this technique is most frequently used on islands (e.g. Moulton and Pimm 1983,
Smallwood 1994), it may be possible to apply it to non-island nations as well.
In summary, therefore, many (but not all) of the hypotheses that have been
formulated to explain the success of certain species rely on comparisons with other
species and cannot be employed in a single-species system. The focus of this chapter will
be twofold; (1) create a minimum adequate General Linear Model (GLM) for population
size, based upon abiotic factors, patterns of morphological dispersion, numbers of
individuals released and number of attempts made, and area; and (2) create a binary
logistic regression function to separate countries where Rose-ringed Parakeets have
become naturalized from countries where Rose-ringed Parakeets failed to establish a self-
sustaining population.
METHODS:
A literature review was undertaken to determine the number of countries where
Rose-ringed Parakeets bred (as Rose-ringed Parakeets are so popular as pets, it seems
likely that introductions have occurred in nearly every country across the world, so
evidence of breeding was used instead). Note that evidence of breeding does not
necessarily indicate that Rose-ringed Parakeets have become naturalized, as it is possible
for them to breed once or twice and still fail to establish a self-sustaining population. A
population was defined as successfully naturalized if evidence for breeding persisted into
the present day.
92

For each country, data on average ground-frost frequency (the number of days per
month with the minimum temperature < 0 ºC) for the period 1961-1990 were collected
from IDI/LDEO (2002). Cold weather is the only abiotic factor that has been suggested in
the literature to have a detrimental effect on Rose-ringed Parakeets (Roscoe et al. 1976),
and the number of days on which the minimum temperature dipped below 0 ºC is taken to
indicate the amount of exposure that the species has to cold weather.
The influence of morphological dispersion was evaluated by counting the
numbers of psittaciform species established in each country area where Rose-ringed
Parakeets were introduced. The numbers of native parrot species in each location were
gathered from Forshaw (1989) and Juniper and Parr (1998) while the numbers of
introduced parrot species at each location were given by Long (1981) and Lever (1987).
Information on the area of each country or island was collected from the Central
Intelligence Agency (CIA) World Factbook (2002). No information was available about
the number of individuals released and/or the introduction effort. However, parakeets are
frequently kept as pets, and so human population size for each country was used as a
surrogate for introduction effort, using data from the CIA World Factbook (2002).
Human population size is at best a rough estimate for introduction effort, as the
proportion of people keeping birds in each country will undoubtedly vary. It is also
possible that the larger the human population, the greater the potential food sources
available to the parakeets, both from introduced plants as well as feeders.
For each country, the length of time between the first reported nesting attempt and
a population estimate was determined from the available literature. (This determines
whether the population has successfully naturalized). Locations where Rose-ringed
93

Parakeets successfully established populations, as well as locations where Rose-ringed
Parakeets failed to establish populations were included, as per the recommendations of
Usher (1988). The data were tested for normality, and were log-transformed if necessary.
A balanced GLM for predicting population size was constructed using elapsed
time since first reported breeding, number of native parrot species, number of introduced
parrot species, number of frosts, whether a country was an island or a continent, and
country area, as well as interactions between these variables. In order to create a
minimum adequate model, variables with a p-value greater than 0.1 were eliminated
through backwards deletion. (This relatively liberal p-value was chosen due to the limited
sample size.)
A binary logistic regression was created in order to identify locations where Rose-
ringed Parakeets established populations and locations where they failed to become self-
sustaining. The variables used initially in the GLM were standardized and then included.
Backwards deletion of variables was carried out in order to construct a minimum
adequate model.
All statistics were calculated with SPSS v.11 and results are presented as mean ±
standard deviation except where noted.
RESULTS:
Rose-ringed Parakeets were found to have bred in 35 countries, although
populations are extinct in three of these countries (see Table 1). Unfortunately, however,
information on current population sizes in many countries is nonexistent. A literature
review provided complete data for only 15 countries. Four widely separated locations in
94

the US reported breeding Rose-ringed Parakeets, and this increased the sample size to 18.
However, information on failed populations of Rose-ringed Parakeets was limited, with
only four locations reported (Iraq, Kenya, Tanzania, and New York City, USA).
It was not possible to create a significant minimum adequate GLM for predicting
population size (n = 18, df = 18, R 2 = 0.322, p = 0.688; see Table 2 for full statistics).
Backwards deletion of variables resulted in the GLM containing the number of frosts per
year as the only remaining variable (n = 18, df = 18, R 2 = 0.127, p = 0.147; see Table 3).
Similarly, it was not possible to create a significant logistic regression to separate
locations where introduced Rose-ringed Parakeets became established from locations
where Rose-ringed Parakeets failed to become established (n = 38, χ 2 = 9.020, d.f. = 7,
Nagelkerke R 2 = 0.431, p = 0.251; see Table 4). Backwards deletion of variables resulted
in the binary logistic regression containing the date of introduction as the sole variable (n
= 38, χ 2 = 3.115, d.f. = 1, Nagelkerke R 2 = 0.161, p = 0.078; see Table 5). The logistic
regression correctly classified 34 of the 34 naturalized populations (100%), but was
unable to correctly classify any (0%) of the extinct populations (0 of 4 populations
correctly classified).
However, it was possible to create a significant discriminant function to separate
successful introductions from unsuccessful introductions. A discriminant function
utilizing all 7 variables (numbers of native parrot species, numbers of introduced parrot
species, date of introduction, numbers of frosts per year, whether the country was an
island or continent, the human population size, and the area was not significant (Wilks’
Lambda = 0.737, χ 2 = 9.930, df = 7, p = 0.193; see Table 6). However, it was possible to
create a significant discriminant function using backwards deletion (Wilks’ Lambda =
95

0.764, χ 2 = 9.306, df = 3, p = 0.025; see Table 7). This discriminant function included the
date of introduction, the numbers of introduced parrot species, and the area. It correctly
predicted 3 of 4 extinct populations (75%) and 32 of 34 established populations (94.1%).
Overall, it correctly classified 92.1% of all cases (n = 38; see Table 7).
DISCUSSION:
Although unsuccessful introductions are crucial for statistical evaluation of
variables, there is an inherent bias towards recording successful invasive species, as
successful invasive species are reported in the literature while unsuccessful invasive
species are seldom documented (Usher 1988). Consequently, it is not surprising that the
GLM and the binary logistic regression were not significant, as the number of failed
introductions recorded in the literature (n = 4) is very small.
The results of the discriminant function reveal that the date of introduction, the
numbers of introduced parrot species, and the area were important in determining
whether Rose-ringed Parakeets became naturalized. However, it should be noted that this
is based on only four reported failed introductions, and so these results should be
interpreted with caution.
The date of introduction coefficient was positive (see Table 7) indicating that
Rose-ringed Parakeets were more likely to become naturalized if released towards the
end of the twentieth century than at the beginning. The mean date of introduction for
populations that failed to become established was 1953 ± 22 years, while the mean date
of introduction for populations that became established was 1970 ± 25 years.
96

The reasons for this trend are unknown. Rose-ringed Parakeets are a long-lived
species, and may attain 34 years of age (Brouwer et al. 2000). It is possible that Rose-
ringed Parakeet populations that are listed as established within the past 20-30 years in
the literature may consist of non-reproducing adult birds, whose presence for many years
may give the false impression of an established population. It is also possible that Rose-
ringed Parakeets may have become more popular as pets during the last few decades of
the twentieth century, resulting in greater numbers of inadvertently released birds and
hence more locations where they could become naturalized.
The numbers of introduced parrot species was also part of the significant
discriminant function. This positive coefficient (see Table 7) suggests that if other parrot
species can become naturalized in an area, then it is likely Rose-ringed Parakeets can
become established as well. Areas where Rose-ringed Parakeets became naturalized had
an average of 0.5 ± 1.3 introduced parrot species, while areas where Rose-ringed
Parakeets failed to establish self-sustaining populations had an average of 0.3 ± 0.5
introduced parrot species. Interestingly, this is contrary to the effect predicted under the
morphological dispersion hypothesis (Moulton and Pimm 1983, Duncan and Blackburn
2002).
Finally, area was inversely related to the probability of establishing a population
(see Table 7). The negative coefficient for area suggests that Rose-ringed Parakeets are
more likely to become established on smaller landmasses than on larger landmasses. This
is consistent with the idea of biotic resistance. Smaller landmasses (such as islands) may
have fewer species on them, which may increase the probability of an introduced species
becoming naturalized (Moulton and Pimm 1983).
97

Given that Rose-ringed Parakeets suffer from frostbite during the winter in
northern locations (Roscoe et al. 1976), it is surprising that the number of frosts per year
was relatively unimportant. It is possible that this result is due to the limited number of
failed introduction attempts documented in the literature. It may be that Rose-ringed
Parakeets introduced into more northerly locations died during the winter and were never
recorded as being introduced. For instance, Rose-ringed Parakeets have been seen in
Chicago, Illionis (USA) on at least one occasion (Perrins pers. comm.) If more
information on failed introductions into northern locations was available, it is possible
that the severity of winter weather would be even more significant.
Although it was not possible to examine quantitatively the majority of hypotheses
pertaining to the probability of a species becoming established, it is possible to examine
many of these hypotheses qualitatively. Greater reproductive potential is one factor that
may allow a species to become established (e.g. Veltman et al. 1996). Rose-ringed
Parakeets in their native range fledge 1.4 – 3.0 young per nest (Lamba 1966,
Shivanarayan et al. 1981, Hossain et al. 1993). This reproductive output is greater than
that reported for larger parrots (e.g. Merritt et al. 1986, Lloyd and Powlesland 1994), but
is less than that reported for smaller parrots (e.g. Spreyer and Bucher 1998).
How widespread and abundant a species is has been positively correlated with its
chances of becoming established in a new location (Williamson 1993, Goodwin et al.
1999). Qualitatively, Rose-ringed Parakeets fit these criteria very well and are reported
to be widespread and abundant in their native range (Juniper and Parr 1998). The Rose-
ringed Parakeet has the widest range of any Old World parrot; it is found on the Indian
subcontinent as well as sub-Saharan Africa (Forshaw 1989, Juniper and Parr 1998).
98

Because Rose-ringed Parakeets frequently feed in great numbers on cultivated grains and
fruits they are considered a serious crop pest in India (Reddy 1998, Mukherjee et al.
2000).
Commensalism with humans is another trait that is believed to enhance the
probability of an introduced species becoming established (Lockwood 1999, Cassey
2002). Rose-ringed Parakeets use human-modified habitats for feeding (e.g. farms – see
above), and it is believed that the increase in their populations in recent decades has been
fueled by the spread of farms in Asia (Juniper and Parr 1998). These parakeets will use a
variety of different forests for nesting, but will also occasionally nest in buildings
(Forshaw 1989, Juniper and Parr 1998).
It has been suggested that body size might affect a species’ chances of becoming
established, with larger animals (within a genera or family) having a greater probability
of naturalizing successfully. Rose-ringed Parakeets measure 38-42 cm in length (Forshaw
1989, Juniper and Parr 1998) and may weigh from 120 to 160 g (Ali and Ripley 1969,
Roselaar 1985). Relative to other psittaciformes, they are medium-sized in length, but
fairly light with regards to weight (Forshaw 1989, Juniper and Parr 1998). Within the
genus Psittacula, they are mid-sized. They are larger than several species, such as the
Blossom-headed Parakeet Psittacula roseate and the Red-breasted Parakeet Psittacula
alexandri but are smaller than the Derbyan Parakeet Psittacula derbiana.
Dietary generalism is another life-history trait thought to correlate positively with
a species’ probability of establishment. Rose-ringed Parakeets consume a broad range of
plant matter, including Red Gram Cajanus cajan, Green Gram Phaseolus aureus, Black
Gram P. mungo, Bengal Gram Cicer arietinum, Sunflower Helianthus anuus, Safflower
99

Carthamus tinctorius, Rice Oryza sativa, and Maize Zea mays (Rao and Shivanarayan
1981). In India, Rose-ringed Parakeets primarily consumed Guava Psidium guajava seeds
from January to March, Mulberry Morus sp. seeds during April and May, a variety of
seeds during June, Guava seeds during July and August, and a variety of cereal seeds
(including pearl millet Pennisetum glaucum, sorghum Sorghum bicolour, and maize)
from August to December (Saini et al. 1994).
Rose-ringed Parakeets introduced into the UK have been observed feeding on a
variety of food items. The following summary was compiled by Roselaar (1985), who
wrote…“In Britain, food items recorded include fruits and berries of various Rosaceae:
apples and crab apples Malus, pears Pyrus, rose Rosa, hawthorn Crataegus, Cotoneaster,
cherries and plums Prunus, raspberries Rubus, strawberries Fragaria; also berries of
holly Ilex and elder Sambucus, grapes, peeled bananas, sliced oranges, peas Pisum, barley
Hordeum, maize Zea, peanuts Arachis hypogaea, beech mast Fagus, Horse Chestnuts,
Aesculus hippocastanum, and seeds of Hornbeam Carpinus betulus, Ash Fraxinus
excelsior and pine Pinus; also takes bread, bacon rind, biscuits, corn, and meat (from
bone on tree).” In addition, they have been observed feeding on sunflower seeds, peanuts,
miscellaneous tree buds and flowers, and mistletoe Viscum sp. (pers. obs.).
Although no information is available in the literature regarding the population
demography of Rose-ringed Parakeets, it appears likely that this species has fairly low
demographic stochasticity. Rose-ringed Parakeets commonly live for 20 years in
captivity (Pithon and Dytham 1999) and can live for up to 34 years (Brouwer et al. 2000).
Adult survival in other long-lived parrots tends to be high as well (Meyers et al. 1996,
Clout and Merton 1998, Sanz and Grajel 1998), leading to low demographic
100

stochasticity. In addition, the popularity of this species as a pet means that multiple
releases in multiple areas occur, a scenario that reduces the likelihood of all populations
undergoing stochastic extinction (Duncan et al. 2001).
In conclusion, neither a significant minimum adequate GLM nor a significant
binary logistic regression could be created, due perhaps to the limited number of failed
introductions recorded in the literature. However, a significant discriminant function was
created which was generally successful in separating countries where Rose-ringed
Parakeet populations succeeded from countries where Rose-ringed Parakeet populations
failed to become established. This discriminant function relied upon the date of
introduction, the numbers of introduced parrot species, and the area. It is likely that future
efforts to predict population sizes and probability of naturalization of this and other
species will continue to be hobbled by the bias against reporting failed introductions
unless a concerted effort is made to report these failed introductions.
101

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114

TABLES:
Table 1: A summary of non-native breeding populations of Rose-ringed Parakeets across the world.
Countries Date
introduced
Austria

China 1903 Hong Kong,
Macao

Iraq 1932 Baghdad 0 0 0 22
437,072
1996
Allouse
(Baghdad)
1953, Long
1981, Lever
1987,
Forshaw
1989
Israel 1963 Coast &
eastern
valleys
2000 0 0 13 (Haifa) 20,770 Shirihai 1996
Italy 1970s Widely Unknown 0 1 37 (Naples) 301,230 Spano and
introduced
Truffi 1986,
Lever 1987,
Frassinet et
al. 2001
Japan 1960s Osaka, 800 0 0 117 (Tokyo) 230,500 Lever 1987,
Nagoya,
Tokyo, and
Miyazakiken
(Honshu) Brazil 1991
Jordan

Malaysia

et al. 1996
Seychelles mid 1970s Mahe 30 0 1 0 (Victoria) 156 (Mahe) Millett pers.
Singapore

United
Kingdom
1969 Greater
London, Isle
of Thanet
USA 1964 Southern
California
(1964),
southern
Florida
(1972),
Kaua’i
(1970s),
New York
City (1974)
6000 0 0 86 (London) 229,462
(Great
900
(Southern
California)
32
(southern
Florida)
100+
(Kaua’i)
0 5 1 (Naples)
90 (NYC)
7 (LA)
0 (Kauai)
Britain)
9,428,692
(continental
USA)
1438
(Kauai)
Aspinall
1996
BOU 1983,
Butler 2002
Lever 1987,
Pratt et al.
1987, Denny
1997,
Gimball pers.
comm..
Venezuela 1984 Caracas 60 4 0 0 (Caracas) 912,050 Nebot 1999
Yemen 1962 Aden,
Mocha,
Sanaa
Unknown 0 0 19 (Aden) 527,970 Long 1981,
Lever 1987,
Porter et al.
1996
120

Table 2: A summary of the GLM for predicting Rose-ringed Parakeet population size in
countries where they have been introduced (n = 18). This GLM was not significant (R 2 =
0.322, df = 18, p = 0.688).
Source ß
Type III Sum of
Squares df
Mean
Square F Sig.
Corrected Model 21972013.7 7 3138859.096 0.678 0.688
Intercept 4368.044 621954.465 1 621954.465 0.134 0.722
Area (logtransformed)
758.970 1748903.398 1 1748903.398 0.378 0.552
Number of
frosts/year (logtransformed)
Date of
introduction (logtransformed)
Number of native
parrot species
Number of
introduced parrot
species
1.311 7612901.704 1 7612901.704 1.645 0.229
1502.455 793454.447 1 793454.447 0.171 0.688
260.806 436490.799 1 436490.799 0.094 0.765
-17.713 6125.411 1 6125.411 0.001 0.972
Continent -2074.788 6742153.244 1 6742153.244 1.457 0.255
Number of people
(log-transformed)
-1308.416 3080800.432 1 3080800.432 0.666 0.434
Error 46270833.4 10 46270833.4
Total 95308024.0 18
Corrected total 68242847.1 17
121

Table 3: A summary of the GLM for predicting Rose-ringed Parakeet population size in
countries where they have been introduced created using backwards deletion 1 . This
model is not significant (df = 18, R 2 = 0.127, p = 0.147) and includes only frosts per year
(n = 18).
Source ß
Corrected
Model
Type III Sum of
Squares df Mean Square F Sig.
8634970.132 1 8634970.132 2.318 0.147
Intercept 273.673 466258.431 1 466258.431 0.125 0.728
Number of
frosts/yr (log-
corrected)
0.777
8634970.132 1 8634970.132 2.318 0.147
Error 59607876.979 16 3725492.311
Total 95308024.000 18
Corrected Total 68242847.111 17
1 Variables deleted from the model include date of introduction, the number of native and
introduced parrot species, the human population, the area, whether the site of introduction
was on an island or a continent, and interactions.
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Table 4: A summary of the binary logistic regression used to separate countries where
Rose-ringed Parakeets were successfully naturalized form countries where they failed to
establish self-sustaining populations. This function was not significant (χ 2 = 9.020, df = 7,
Nagelkerke R 2 = 0.431, p = 0.251) and successfully classifies only one of three failed
introductions and 33 of 34 successful introductions.
Numbers of native parrot
species
Numbers of introduced parrot
species
B S.E. Wald df Sig. Exp(B)
-0.541 1.136 0.227 1 0.634 0.582
0.766 0.775 0.976 1 0.323 2.151
Continent 1.038 2.715 0.146 1 0.702 2.823
Area (log-corrected) 0.187 1.242 0.023 1 0.880 1.206
Human population size (log-
corrected)
Date of introduction (log-
corrected)
Number of frosts per year
(log-corrected)
Constant 31.24
-3.158 2.314 1.862 1 0.172 0.043
-4.834 3.826 1.596 1 0.207 0.008
0.000 0.001 0.054 1 0.817 1.000
9
16.89
8
3.420 1 0.064
37246497386126.
510
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Table 5: A summary of the binary logistic regression used to separate countries where
Rose-ringed Parakeets were successfully naturalized from countries where they failed to
establish self-sustaining populations created using backwards deletion 1 . This function
was not significant (χ 2 = 3.115, df = 1, Nagelkerke R 2 = 0.161, p = 0.078) and includes
only the date since the introduction. This table was unsuccessful in classifying any of the
failed introductions (0 of 4 classified correctly), but it correctly classified all the
successful introductions (34 of 34 classified correctly).
B S.E. Wald df Sig. Exp(B)
Date of introduction (log-corrected) -3.039 1.815 2.805 1 0.094 0.048
Constant 6.589 2.892 5.190 1 0.023 727.241
1 Variables deleted from the model include area, frosts per year, the number of native and
introduced parrot species, the area, and whether the site of introduction was on an island
or a continent
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Table 6: A summary of the discriminant function used to separate countries where Rose-
ringed Parakeets were successfully naturalized from countries where they failed to
establish self-sustaining populations. This function was not significant (Wilk’s Lambda =
0.737, χ 2 = 9.930, df = 7, p = 0.193). It correctly classified three of the four (75%) failed
introductions, but only correctly classified 30 of 34 (88%) successful introductions.
Standardized Canonical Discriminant Function
Coefficients
Numbers of native parrot species -0.143
Numbers of introduced parrot
species
1.524
Date of introduction 0.467
Numbers of frosts per year 0.283
Continent -0.195
Human population size -0.172
Area -1.501
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Table 7: A summary of the discriminant function used to separate countries where Rose-
ringed Parakeets were successfully naturalized from countries where they failed to
establish self-sustaining populations using backwards deletion 1 . This function is
significant (Wilks’ Lambda = 0.764, χ 2 = 9.306, df = 3, p = 0.025) and includes the date
of introduction, the numbers of introduced parrot species, and the area. This function
correctly classified 3 of 4 extinct populations (75%) and 32 of 34 established populations
(94.1%).
Standardized Canonical Discriminant Function
Coefficients
Date of introduction 0.609
Numbers of introduced parrot
species
1.333
Area (km 2 ) -1.484
1 Variables deleted from the model include frosts per year, the number of native parrot
species, the human population, and whether the site of introduction was on an island or a
continent
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Chapter 4:
Breeding success in the UK
ABSTRACT:
Rose-ringed Parakeets began breeding in the UK in 1969. Their population slowly
grew to 1500 individuals by 1996; thereafter the population increased rapidly to an
estimated 5800 birds in 2001. The only published study on their breeding biology in the
UK suggests that they have poor reproductive success, fledging only 0.8 young per nest.
Given the rapid increase in the population, it is likely that their reproductive success is
higher than the published estimate. During 2001-2003, a total of 108 nests were located
and monitored in southwest London, southeast London and the Isle of Thanet. Trees
where parakeets chose to breed had a larger diameter at breast height (dbh) and a greater
amount of potentially concealing trees and shrubs within 10 m than randomly sampled
trees. The date of first egg was 27 February and the median clutch size was 4 eggs.
Clutch size was inversely related to tree height, perhaps because the largest clutches were
on the Isle of Thanet where tree size tended to be shortest. Reproductive success was 1.9
± 0.1 young fledged per nest, which was considerably higher than the previous estimate
of reproductive success (0.8 young fledged per nest) reported by Pithon and Dytham
(1999). Reproductive success was linked to the number of years the nest was occupied
and the clutch size. Although it has been suggested that nest cavities are not limiting for
the Greater London population, nest cavities may be limiting for the Isle of Thanet
population, as nest box usage was much higher in this population than had been reported
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in the Greater London area. Finally, although it has been suggested that Rose-ringed
Parakeets may have a negative impact on native cavity-nesting species, their population
is apparently too small at this time for them to affect native species.
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INTRODUCTION:
The Rose-ringed Parakeet is an established exotic in Britain (BOU 1983). They
were first reported breeding in Britain in 1969 (Hudson 1974, Lever 1977), and their
population slowly increased to an estimated 500 birds in 1983 (BOU 1983) and 1500
birds by 1996 (Pithon and Dytham 1999a). Thereafter, the population rapidly increased to
an estimated 5800 birds by 2001 (Butler 2002). The rapid increase of this introduced
species is a cause for concern from both an economic standpoint (it is a crop pest in its
native range) and from a conservation standpoint (it could potentially outcompete native
species for cavities). Consequently, its breeding biology should be investigated to
determine whether the rapid rate of increase observed during the late 1990s and early 21 st
century will continue.
Two types of study on the breeding biology of birds are widely reported in the
literature; nest-site selection and factors influencing reproductive success. Nest-site
selection studies identify the habitat characteristics that a species requires to breed (e.g.
Glue and Boswell 1994, Jones and Robertson 2001). These studies may examine habitat
characteristics on a variety of scales, from a few square metres (e.g. Hatchwell et al.
1999) to hectares (e.g. Holt and Martin 1997). Such studies are particularly useful for
identifying the habitat requirements of a declining breeder (e.g. Jones and Robertson
2001). They are also useful for evaluating the potential spread of an invasive species (e.g.
Hyman and Pruett-Jones 1996, Burger and Gochfield 2000).
In contrast, studies predicting reproductive success attempt to predict fledging
success on the basis of one or more variables (e.g. age; Wiktander et al. 2001). These
studies are useful for evaluating the relative influence of the variables on reproductive
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success. To date, little work has been done on either nest-site selection or factors that
influence reproductive success in Rose-ringed Parakeets.
The breeding biology of the Rose-ringed Parakeet on the Indian sub-continent is
well known. In southern India, Rose-ringed Parakeets begin nesting as early as December
while in northern India they begin nesting in February (Lamba 1966, Shivanarayan et al.
1981, Forshaw 1989, Juniper and Parr 1998). Clutch size ranges from two to six eggs
(Lamba 1966, Shivanarayan et al. 1981). Rose-ringed Parakeets spend a total of nine to
ten weeks in the nest - three weeks incubating eggs, and six to seven weeks feeding the
young (Lamba 1966, Shivanarayan et al. 1981, Forshaw 1989, Juniper and Parr 1998).
In their Indian range, Rose-ringed Parakeets enjoy a relatively high rate of
breeding success. Lamba (1966) examined 33 nests and found that an average of 3.0
young fledged per nest. Shivanarayan et al. (1981) examined 66 nests and found that an
average of 1.7 young fledged per nest. This lower rate of reproduction was attributed to
predation by crows and snakes (Shivanarayan et al. 1981). A summary of published data
on breeding success in the Rose-ringed Parakeet can be seen in Table 1.
A recent study conducted by Pithon and Dytham examined 12 nests in the Greater
London area during 1997-1998 and found that an average of 0.8 young fledged per nest
(Pithon and Dytham 1999a). This level of productivity is much lower than in India and in
fact may be too low to explain the observed rapid population growth (see Chapter 2), as
the population at the four major roosts has nearly tripled during the five-year period
1996-2001, from 1508 birds to approximately 5800 birds (Pithon and Dytham 1999b,
pers. obs.).
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Nest-site selection
A number of studies have identified variables on which nest-site selection is
based. Some species exhibit a preference in their nest orientation, generally for
thermoregulation reasons. For example, Gila Woodpeckers Melanerpes uropygialis and
Gilded Flickers Colaptes chrysoides in the southwestern US tend to create nestholes that
are oriented towards the north (Inouye et al. 1981, Zwartjes and Nordell 1998). Nests
oriented towards the north tend to remain cooler than nests oriented towards the south. In
contrast, Tree Swallows Tachycineta bicolour prefer nest cavities to be oriented toward
the south in order to increase the temperature in the nest (Rendell and Robertson 1994).
Finally, Orange-breasted Sunbirds Nectarinia violacea in South Africa, which breed
during the austral winter, prefer their nests to be oriented away from the prevailing winds
(Williams 2002)
For some species, nest-site selection may be based on the height of the tree. Monk
Parakeets Myiopsitta monachus in Florida, for example chose to nest in taller trees than
were generally available (Burger and Gochfield 2000). Some cavity-nesting species
prefer larger diameter trees (Burger and Gochfield 2000). For instance, in New Zealand
the Mohua Mohoua ochrocephala and the Yellow-crowned Parakeet Cyanoramphus
auriceps bred most often in trees with diametres >70 cm. (Elliot et al. 1996).
Some species prefer to nest in a specific tree species (Emison et al. 1994, Hooge
et al. 1999, Burger and Gochfield 2000). For instance, Yellow-crowned Parakeets in New
Zealand prefer to breed in Red Beech Nothofagus fusca trees (Elliot et al. 1996).
Finally, nest-site selection may also be based on the habitat immediately around
the nest tree. Hermit Thrushes Catharus gutattus in Arizona, for example, chose to nest
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in trees that were surrounded by small White Firs Abies concolour (Martin and Roper
1988). The increased tree density may reduce predation on the nests. Similarly, Black-
throated Blue Warblers Dendroica caerulescens chose to nest in dense patches of
Hobblebush Viburnum alnifolium (Holway 1991).
Predicting reproductive success
For some species, reproductive success may depend upon nest height (Ludvig et
al. 1995, Sockman 1997). Mohua nests that were farther from the ground were less likely
to be predated (Elliot et al. 1996). Sulphur-crested Cockatoo (Cacatua sulphurea) nests
that were higher off the ground tended to fledge more young, presumably as this reduced
their chances of being depredated by humans (Marsden and Jones 1997). In contrast,
other species (e.g. Long-tailed Tits Aegithalos caudatus) may experience improved
fledging rates with lower nest heights (Hatchwell et al. 1999). Nest height may depend
upon the height of available trees (Brightsmith 1999).
In addition to the factors that affect nest-site selection, other factors may influence
reproductive success. Some species exhibit enhanced reproductive output with age
(Wiktander et al. 2001; but see Newton and Rothery 2002). It has also been suggested
that secondary cavity users that breed in nestboxes may exhibit artificially high fledging
rates (Nilsson 1986). Finally, birds that reuse cavities may face increased predation risk
(Sonerud 1985, 1989).
In this chapter, I will test the following two hypotheses. (1) Nest-site selection is
restricted by the available habitat and the habitat in which they choose to breed is not
random. (2) Reproductive success can be predicted from habitat as well as age of the
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eeding birds. Finally I will attempt to determine whether parakeet nest site choice has
any negative implications for other cavity-nesters that may compete for the same nesting
sites.
METHODS:
From mid-February 2001 to July 2003 extensive nest searching was undertaken in
the Greater London area, around the coast of Kent, and in the town of Studland, Dorset.
Parks and other areas where public access was allowed were searched every other day (all
day) from late February to July. Once potential nests had been located (e.g. where a
female was seen entering or leaving a cavity while a male was present nearby) the
contents of the nest were examined using a miniature black-and-white video camera
attached by a 20m length of cable to a 4" LCD television made by Optimus (similar to the
“burrow probe” reported elsewhere, e.g Enkerlin-Hoeflich et al. 1999). A small ‘Maglite’
torch (Solitaire single cell AAA) was taped to the video camera to increase the
illumination in the cavity. If the nest was less than 7.5 metres above the ground, a ladder
was employed in order to reach the nest. Tree-climbing was employed to reach nests that
were located more than 7.5 metres above the ground.
Nests were visited on a weekly basis, and the number of eggs and/or chicks was
recorded. Inspection of the nests typically caused the adult(s) to leave, although if the
female were in the cavity she would often refuse to budge. The adults returned shortly
after nest inspection ceased. Chicks were determined to have fledged if they were no
longer in the nest after seven weeks.
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In late June or July (after the young birds had fledged) a series of measurements
was taken of the nesting tree. This included recording the tree species, the type of cavity
(whether natural, woodpecker, or nestbox), the height of the tree (using a clinometer), the
nest height (using a clinometer), orientation of cavity (using a compass), and dbh
(diameter of the tree at breast height which is standardized as 1.4 m). In addition, the
species was determined and the dbh measured for all trees within 10m of the nest cavity
(a tree was defined as a plant with a dbh of 3.0 cm or more). Finally, the measurements
(with the exception of cavity orientation) were repeated 100m away in a random
direction. If no tree was located at the random location, then the general habitat type was
noted, but the location was not included in the analysis.
In an effort to determine whether nest cavities on the Isle of Thanet were a
limiting factor, a total of 34 nestboxes were put up during June and July of 2002.
Although subadult males were observed breeding in the Greater London area, only adult
males (which were presumably more dominant) were observed breeding on the Isle of
Thanet, which may suggest that nest cavities are limiting. In addition, nestbox usage in a
previous study was very low (1 out of 175 nestboxes used; Pithon and Dytham 1999a), so
if a greater percentage of nestboxes was used on the Isle of Thanet, this may suggest that
nest sites there are limiting. The nestboxes were based on the same measurements as
those put up by a previous study in 1997 (Pithon and Dytham 1999a). In the Pithon and
Dytham (1999a) study, nestboxes were placed approximately 8 metres above the ground
in trees with a diameter greater than the nestboxes, and so nestboxes in this study were
placed approximately 8 metres above the ground in trees with a diameter greater than the
nestbox. Nestboxes were placed in six public areas on the Isle of Thanet – three parks
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where parakeets had previously been found breeding, and three parks where parakeets
had not been noted breeding.
A balanced General Linear Model (GLM) for predicted fledging success was
constructed using clutch size, dbh (measured with a measuring tape), nest height
(measured with a clinometer), tree height (measured with a clinometer), number of years
that a nest had been occupied (since the beginning of the study), male age (based on the
presence or absence of a pink neck ring), year, region, orientation of the nest cavity
(measured with a compass), type of cavity (natural, woodpecker, or nestbox), amount of
understory (measured both by counting all trees within 10 m [basal stem count], as well
as summing the diametres of all trees within 10 m [basal area]), date of first egg (back-
calculated if necessary from the hatching date), and interactions. In order to create a
minimum adequate model, variables with a p-value greater than 0.05 were eliminated
through backwards deletion.
All statistical analyses were performed using SPSS 11.0. Data are presented as
mean ± standard deviation except where noted.
RESULTS:
Nest site choice
In total, 108 nests were located over the course of the study (33 in 2001, 37 in
2002, 38 in 2003; see Appendix 1), consisting of 46 nests on the Isle of Thanet, 34 in SE
London, 26 in SW London and two nests at Studland (Dorset; see Figures 1 and 2).
Nests were located an average of 8.1 ± 3.8 m off the ground. Significant
differences in nest height, tree height, dbh, basal area of surrounding shrubs and trees,
135

and the number of stems of surrounding shrubs and trees were found between the four
regions (Kruskal-Wallis tests; see Table 2). Trees that parakeets chose to breed in were
shorter on the Isle of Thanet with correspondingly lower nest heights and smaller
diametres (see Table 2).
Tree height and basal stem counts did not differ significantly between trees where
Rose-ringed Parakeets chose to nest and randomly sampled trees 100 m away (p > 0.09;
see Table 3). Similarly, the orientation of the cavities that parakeets chose to nest in did
not differ from random (runs test, Z = 0.786, p = 0.432). However, significant differences
were found in dbh (p = 0.002) and basal area (p = 0.003; see Table 3). Parakeets chose to
nest in trees with a larger dbh (73.7 ± 34.1 cm) than randomly sampled trees (46.7 ± 30.8
cm; see Table 3). In addition, parakeets chose to nest in trees that had greater shrub and
tree cover surrounding them (basal area = 233.9 ± 174.6 cm) than randomly sampled
trees (basal area = 153.6 ± 186.6; see Table 3).
Parakeets nested in trees of twelve different genera, with Fraxinus accounting for
33% of the trees used and Quercus accounting for 22% (see Table 4), despite the fact that
Acer accounted for 37% of the random trees sampled (see Table 5). Parakeet nesting
attempts were found primarily in old woodpecker nests (n = 58), but were also found in
natural cavities (n = 31) and nestboxes (n = 19). Parakeets did not apparently excavate
their own cavities, and only one instance of enlarging an existing cavity (a Great Spotted
Woodpecker hole) was noted. Nest-site reuse was highest on the Isle of Thanet (see
Figure 3). During the summer of 2002, a total of 34 nestboxes were put up on the Isle of
Thanet, and 23 of these boxes were still present during the summer of 2003. Six of the
nestboxes (26.1%) were used by parakeets in 2003. In stark contrast to this, only 0.6% of
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nestboxes were used (one out of 175) in the Greater London area during 1998 (Pithon and
Dytham 1999a).
Factors affecting reproductive success
Eggs were first recorded on 5 March, but may have been laid as early as 27
February (back-calculated from hatching date; see Figure 4). A few parakeets laid eggs
through mid-May (see Figure 4). Clutch sizes ranged from one to seven eggs, with a
mean of 3.7 ± 1.2 eggs per clutch (median = 4, n = 77; see Figure 5). There was no
difference in clutch size between Southwest London, Southeast London, and the Isle of
Thanet (Kruskal-Wallis, χ 2 = 3.408, df = 2, p = 0.182).
The estimated (mean ± s.e.) average number of birds fledged was 1.9 ± 0.1 chicks
(see Figure 6). There were no significant differences in fledging rates between regions
(see Table 6; Kruskal-Wallis, χ 2 = 0.793, d.f. = 3, p = 0.851). Similarly, there was no
significant difference in fledging rates between years (Kruskal-Wallis, χ 2 = 0.583, d.f. =
2, p = 0.747).
A significant GLM (p < 0.001) was created to predict fledging success based on
clutch size and the number of years that the nest cavity was occupied (see Table 7). Both
variables in the GLM were highly significant; clutch size (p = 0.003) was a strong
predictor of fledging success, as was the number of years a nest was occupied (p =
0.002). Neither Dbh, nest height, tree height, male age, year, region, orientation of the
nest cavity, type of cavity (natural, woodpecker, or nestbox), amount of understory, nor
date of first egg or any of their interactions had a significant effect upon fledging success.
137

A significant GLM (p = 0.003) was created to predict clutch size based upon tree
size (see Table 8). Fledging rates were higher when a cavity was reused (see Figure 7)
but there is no evidence that the same birds were reusing the cavities. Surprisingly,
cavities that failed to produce any chicks were nearly as likely to be reused as cavities
that succeeded in fledging chicks (see Figure 8).
Although it has been suggested that birds breeding in nestboxes may have higher
fledging rates than birds breeding in natural cavities (Nilsson 1986), this was not the case
in this study. Although there were differences between fledging rates (mean ± se) for
Rose-ringed Parakeets breeding in nestboxes (1.6 ± 0.4, n = 19), parakeets breeding in
natural cavities (1.8 ± 0.2, n = 31), and parakeets breeding in old woodpecker cavities
(2.2 ± 0.2, n = 51), the differences in fledging rates were not significant (Kruskal-Wallis,
χ 2 = 2.725, df = 2, p = 0.256).
During this study, subadult male Rose-ringed Parakeets (n = 15) were observed
breeding, a result not reported elsewhere. Subadult males (which lacked a rose-coloured
ring) had smaller clutches (3.3 ± 1.3 eggs) than adult males (3.8 ± 1.2 eggs), although the
differences in clutch size were not significant (t-test, df = 75, p = 0.157). Similarly,
subadult males fledged fewer young (1.4 ± 1.2) than adult males (1.9 ± 1.3), although this
difference was not significant (t-test, df = 106, p = 0.135).
DISCUSSION:
Nest site choice
138

Relatively little information about nest-site selection in the Rose-ringed Parakeet
is available in the literature. The mean nest height in this study of 8.5 ± 4.1 m is higher
than the mean nest height of 3.44 ± 0.38 m (number of nests sampled is unknown)
reported in Punjab, India (Simwat and Sidhu 1973). However, it was comparable with the
nest height reported for California where two Rose-ringed Parakeets bred in a Silver
Maple Acer saccharinum at a height of approximately 10 m (Mabb 1997). Nest height
has been correlated with breeding success in some species (Negro and Hiraldo 1993,
Ludvig et al. 1995, Marsden and Jones 1997, Sol et al. 1997), but nest height was not
significant in this study.
All the trees that parakeets chose to nest in were large, with an average height of
19.5 ± 8.1 m (n = 70) and an average dbh of 73.7 ± 34.1 cm (n = 69, note that one tree
was pollarded during the course of the study, altering its height but not its width). The
dbh of trees used in this study is smaller than those trees used in northern India, which
were 120 ± 6 cm (n = 15; Simwat and Sidhu 1973). Tree heights were not reported in
breeding studies of this species in its native range, and so no comparison is possible.
Similarly, no information is available to determine whether parakeet nest-site selection is
based on the amount of cover provided by nearby shrubs and trees.
Some species exhibit a preference in their nest orientation, generally for
thermoregulation reasons. For example, Tree Swallows Tachycineta bicolour prefer nest
cavities to be oriented toward the south in order to increase the temperature in the nest
(Rendell and Robertson 1994). Although it might be expected that the Rose-ringed
Parakeets breeding during the cool British spring would prefer southerly-facing (and
presumably warmer) cavities, the orientation of cavities used did not differ significantly
139

from random. The reasons for this are unknown, but perhaps there were insufficient
numbers of south-facing cavities for the parakeets to choose from, or perhaps at this
latitude cavity orientation did not provide substantial benefits to breeding birds.
Most of the nests found were in Green Woodpecker cavities, which might limit
the spread of this species in Great Britain. Green Woodpeckers are widespread and
common in England and Wales, but are much less numerous in Scotland (Gibbons et al.
1993), and so it is unlikely that Rose-ringed Parakeets will spread into Scotland at the
present time.
Factors affecting reproductive success
The date for the first recorded egg (27 February) is much later than that reported
for southern India (16º N, 21 December, Shivanarayan et al. 1981) but very similar to that
reported in northern India (31º N, 11 March, Simwat and Sidhu 1973) and Bangladesh
(24º N, 10 March, Hossain et al. 1993). It is possible that the inter-country differences in
first egg dates may be due to latitude or daylength. The first recorded egg in this study
was 15 days earlier than a previous study on the breeding habits of this parakeet in the
UK (Pithon and Dytham 1999a). Although second clutches have been reported in other
populations (Hossain et al. 1993), no second clutches were found over the course of this
study.
Mean clutch size (3.7 ± 1.2 eggs) during this study was similar to the mean clutch
size reported elsewhere (Lamba 1966, Simwat and Sidhu 1973, Shivanarayan et al. 1981,
Hossain et al. 1993, Pithon and Dytham 1999a; see Table 4). The range of clutch size (1-
7 eggs) is similar to the values reported elsewhere, although no other study reported a
140

clutch size of one or seven. This nest was located in Studland, Dorset, which has a very
small population of Rose-ringed Parakeets (< 12 birds). Rather than increasing
exponentially, as the populations in the Greater London area and the Isle of Thanet have
done, this population has remained essentially stable since the 1980s (Morrison 1997).
The relatively low rate of reproduction (1.5 ± 0.5 chicks fledged per nest; n = 2) observed
at Studland may be due to inbreeding depression caused by the limited gene pool.
The estimate of 1.9 ± 0.1 young fledged per nest during 2001-2002 was
comparable to that found in Bangladesh and southern India (Shivanarayan et al. 1981,
Hossain et al. 1993), but considerably lower than that reported elsewhere in India (Lamba
1966; see Table 1). Given that Lamba’s (1966) estimate of fledging success is roughly
twice that reported in Bangladesh and southern India (Shivanarayan et al. 1981 Hossain
et al. 1993), it is possible that predation rates were lower at his study sites.
The estimate of 1.9 ± 0.1 young fledged per nest during 2001-2003 was more than
double the 0.8 birds fledged per nest (in the UK) reported previously (Pithon and Dytham
1999a). Reproductive success was higher in all regions during 2001-2003 and was
particularly pronounced in SE London (see Table 9). In 1998, Pithon and Dytham (1999)
estimated that breeding success in SE London was only 0.25 young fledged per nest (n =
4). In contrast, during 2001-2003, breeding success in SE London averaged 2.0 ± 0.2 (n =
34) young fledged per nest.
The GLM revealed that fledging success depended upon the clutch size and the
number of years that a nest was occupied. Surprisingly, nest cavities that were used in
successive years experienced better reproductive success than cavities that were only
used for one year (Figure 7). In theory, birds that reuse nest cavities should be at a greater
141

isk of predation (Sonerud 1985, 1989) and so fledging rates should be lower when nest
cavities are used for successive years. However, nest predation was only infrequently
observed (n = 5), and in all instances was caused by Gray Squirrels Sciurus carolinensis.
(All other instances of egg failure were due to unhatched eggs.) It may be that the
increase in reproductive success noted at reused cavities may be due to age-related
reproductive success (Laycock 1982, Saether 1990, Pärt 2001).
Those birds whose nests were predated should be less willing to reuse that nest in
future years (Sonerud 1985). However, Rose-ringed Parakeets reused nest cavities that
failed to produce any young as often as they reused nest cavities that were successful in
producing young (see Figure 8), although there is no evidence that the same birds reused
the cavities.
Although it has been suggested that birds breeding in nestboxes may fledge more
young than birds breeding in natural cavities (Nilsson 1986) other authors have not been
able to detect a difference (e.g. Miller 2002). Over the course of this study, no differences
in fledging rates were detected between parakeets breeding in natural cavities,
woodpecker cavities, and nestboxes.
Human predation on nests (i.e. removing young from cavities) is a widespread
problem for other parrot species (e.g. Thomsen and Mulliken 1991, Koenig 2001,
Martuscelli 2003). However, it did not appear to be a widespread problem in the UK,
with the exception of the Isle of Thanet. There, several trees had large notches cut into
them. These notches were typically centred on old Green Woodpecker cavities,
presumably to make it easier to remove nestlings. However, although human predation
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may occur on the Isle of Thanet, fledging rates were similar to rates in the Greater
London area.
The discovery that subadult males are breeding in the UK was unexpected.
However, Lamba (1966) refers to a nest of two females. It is possible that one of these
birds may actually have been a subadult male. In addition, an examination of the Rose-
ringed Parakeet specimens at Tring (see Chapter 5) reveals that there is one subadult
specimen there with a slightly enlarged testis, which may indicate that subadult male
Rose-ringed Parakeets occasionally breed in India. The poor breeding success of these
birds relative to that of adult males is consistent with age-specific effects reported
elsewhere (Laycock 1982, Saether 1990, Pärt 2001).
As discussed in Chapter 2, populations in the UK are not increasing at an equal
rate. The population on the Isle of Thanet is increasing more slowly than that in the
Greater London area. From 1996-2001 the Thanet population increased from 299
individuals to 540 individuals (a 181% increase). During the same period the population
in Greater London increased from 1300 individuals to more than 5400 individuals (a
415% increase).
Initially, it appeared possible that Rose-ringed Parakeets breeding along the coast
of Kent may be nesting in sub-optimal habitat, as trees were lower, with smaller
diametres, and the nests were closer to the ground. However, fledging rates on the Isle of
Thanet are comparable with those in SW London and SE London. It seems likely that
other factors such as higher post-fledging mortality or a limited number of suitable nest
sites may be limiting the population growth. However, it is unlikely that higher post-
fledging mortality could be due to predation by raptors as Gibbons et al. (1993) show that
143

aptor densities on the Isle of Thanet are not higher than they are in the Greater London
area.
Although a previous study on the breeding habits of Rose-ringed Parakeets in the
UK concluded that cavities were not a limiting factor in the Greater London area (Pithon
and Dytham 1999a), it appears that they may be a limiting factor on the Isle of Thanet.
Parakeets used six of the 23 nestboxes (26%) present in 2003. In contrast, parakeets bred
in only one of 175 nestboxes (0.6%) in Greater London during 1998 (Pithon and Dytham
1999a)and only two of 137 (1.4%) by 1999 (Baxter 1999), which is still a considerably
lower percentage than on the Isle of Thanet. In the future, it would be worthwhile to place
nestboxes simultaneously both on the Isle of Thanet and in the Greater London area in
order to control for temporal variability while examining whether nestbox usage varies
between locations.
Effects on native species
Previous literature has suggested that as the numbers of parakeets increase, native
species will begin to suffer (e.g. England 1974, Tozer 1974). Rose-ringed Parakeets begin
breeding earlier in the season than most native birds, and so it has been theorized that
parakeets will be able to claim nest cavities first (Lever 1977).
The list of native species which may be vulnerable to competition with parakeets
includes Kestrel Falco tinnunculus, Stock Dove Columba oenas, Green Woodpecker
Picus viridis (which reuses 20% of its old cavities [British Trust for Ornithology 2000]),
Jackdaw Corvus monedula, and European Starling Sturnus vulgaris. Although Rose-
ringed Parakeets were observed examining Great Spotted Woodpecker Dendrocopos
144

major cavities, these were apparently too small for the parakeet to fit inside. Parakeets
were observed on three occasions attempting to widen a Great Spotted Woodpecker hole,
but only once were these efforts ultimately successful. In 2001 and 2002 Rose-ringed
Parakeets were observed attempting to enlarge a Great Spotted Woodpecker cavity on the
Isle of Thanet, but with no success. However, during 2003, the cavity had been enlarged
sufficiently that these birds were able to enter the cavity and breed.
In 2001, a summary of the population changes between 1994 and 2000 in species
surveyed by the BBS (BTO - Breeding Bird Survey) was released (Noble et al. 2001).
European Starling exhibited a significant (p < 0.05) decline (-27%, n = 268) in
southeastern England. Kestrel exhibited a nonsignificant (p > 0.05) decline (-23%, n =
93) in southeastern England, as did Stock Dove (-9%, n = 134). Green Woodpecker, in
contrast, exhibited a significant (p < 0.05) increase (31%, n = 188) over this period, as
did Jackdaw (33%, n = 213).
However, Rose-ringed Parakeets were not listed in this report, presumably
because they did not fit the BTO’s criteria, i.e. that the species be “recorded in at least 20
squares in 2000 in two or more of the nine Regional Development Agency (RDA)
regions of England” (Noble et al. 2001). Despite the fact that parakeets have increased
from approximately 1500 individuals in 1996 (Pithon and Dytham 1999b) to 5800
individuals by 2002 (Butler 2002), neither the BBS nor the CBC (Common Bird Census)
plots have enough breeding parakeets to track changes in the population (J. Marchant,
pers. comm.). This suggests that parakeets are unlikely to be causing the declines
observed in Kestrels, Stock Doves, and Starlings in southeastern England.
145

Although Rose-ringed Parakeets will nest in rock crevices and buildings in their
native range (Ali and Ripley 1969, Roberts 1991, Juniper and Parr 1998), all the nests
found both in this study and those reported by Pithon and Dytham (1999) were in trees,
suggesting that rock crevices and buildings are not used in the UK. If so, this may reduce
the competition with native species as Kestrels, Stock Doves, Jackdaws, and Starlings.
Green Woodpeckers, however, are dependent upon trees, and so the potential
exists that an increase in parakeets may have a negative impact on this species. A survey
of 243 Green Woodpecker nests in the UK found that the trees that Green Woodpeckers
chose to nest in were primarily oaks Quercus spp. and Ash Fraxinus excelsior (Glue and
Boswell 1994), which were the same trees that Rose-ringed Parakeets tended to nest in.
However, parakeets do not appear to be claiming all Green Woodpecker cavities, but
instead tend to occupy higher cavities. The study by Glue and Boswell (1994) reported
that the mean Green Woodpecker cavity height was 4.6 m. In contrast, it was found that
parakeets using Green Woodpecker cavities selected higher cavities for breeding (9.4 ±
4.2 m).
Finally, over the course of the study, it was discovered that Rose-ringed Parakeets
defend only their nest cavity rather than the entire tree, a result consistent with that
observed in India (Lamba 1966). During 2001-2003, parakeets were observed sharing
their nesting tree with European Starlings (n = 5), Stock Dove (n = 1), and Jackdaw (n =
2).
However, there is a definite need for a long-term study on how this species may
affect native species as the population continues to grow. Ideally, long-term monitoring
of cavity-nesting species should begin just beyond the periphery of the Rose-ringed
146

Parakeet’s range in the UK, where parakeets are absent. Then, when parakeets begin
invading the area, changes in the populations of native cavity-nesting species could be
correlated with the increase in parakeet numbers, and any negative interactions between
parakeets and native species could be documented.
In conclusion, Rose-ringed Parakeets prefer to breed in large, wide trees with
shrubs and trees nearby providing cover. Clutch size is related to tree height, while
fledging success depends upon clutch size and the number of years a site was occupied.
Nest sites on the Isle of Thanet may be limiting the ability of the population to increase,
as nestbox usage was higher in this locality than had been reported in SW London and SE
London. Finally, although the population of Rose-ringed Parakeets is increasing rapidly,
information in the literature suggests that they are not yet having an impact on native
species. If the population of Rose-ringed Parakeets continues to increase however, the
potential exists for them eventually to have a negative impact on native cavity-nesting
species.
147

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153

TABLES:
Table 1: A summary of previous studies on the fledging rates of Rose-ringed Parakeets.
Location Number of
nests
Mean clutch No. of young
fledged per
nest
Author
Bangladesh 11 3.6 1.4 (Hossain et al.
1993)
India 24 4.1 3.1 (Lamba 1966)
Southern India 66 3.6 1.7 (Shivanarayan
et al. 1981)
Northern India ? 3.9 ± 0.2 ? (Simwat and
Sidhu 1973)
UK 12 4 0.8 (Pithon and
Dytham 1999a)
154

Table 2: A summary of tree height, nest height, dbh, basal area, and basal stem count in
the four regions were parakeet nests were found. More than one pair occasionally nested
in a tree. One tree at the Isle of Thanet was pollarded in late 2001 or early 2002, leaving
the dbh unchanged but altering the height.
Tree
height
(m)
Nest
height
(m)
Dbh
(cm)
Basal
area
(cm)
Basal
stem
count
SW
London
20.6 ± 6.7
(n = 19)
9.6 ± 3.9
(n = 23)
94.9 ±
27.4
(n = 19)
185.4 ±
119.3
(n = 19)
12.5 ±
10.4
(n = 19)
SE
London
26.2 ± 7.0
(n = 20)
9.6 ± 3.9
(n = 27)
86.3 ±
40.3
(n = 20)
197.802 ±
213.9
(n = 20)
13.1 ±
17.6
(n = 20)
Studland Isle of
19.0 ±
11.4
(n = 2)
6.8 ± 4.3
(n = 2)
37.5 ±
10.3
(n = 2)
214.0 ±
108.0
(n = 2)
6.5 ± 2.1
(n = 2)
Thanet
14.3 ±
5.9
(n = 29)
5.6 ± 2.0
(n = 31)
52.8 ±
17.3
(n = 28)
294.1 ±
168.2
(n = 28)
14.0 ±
11.3 (n =
28)
χ 2 df P
59.890 3 P <
0.001
26.455 3 P <
0.001
54.474 3 P <
23.637 3 P
0.001
10.691 3 P =

Table 3: Characteristics of trees occupied by breeding Rose-ringed Parakeets in southern
England during 2001-2003 and a tree 100 m away in random direction, and univariate
logistic regression parameter estimates. Data include the number of trees sampled (n),
mean ± sd, and range of the variables; results of the logistic regression models are the
estimated coefficient (β), standard error, the univariate Wald statistic and the significance
of the model.
Variable N Mean ±
sd
Tree
Height
Used 70 19.5 ±
8.1
Random 46 14.0 ±
6.3
Dbh Used 69 73.7 ±
34.1
Random 46 46.7 ±
30.8
Basal area Used 69 233.9 ±
174.6
Basal stem
count
Random 46 153.6 ±
186.6
Used 69 13.1 ±
13.0
Random 46 10.2 ±
13.2
Range β se Wald P
5.3 –
39.0
2.7 –
28.0
30.2 –
165.2
3.8 –
130.2
233.9 ±
174.6
153.6 ±
186.6
-0.051 0.037 1.916 0.166
-0.032 0.010 9.986 0.002
-0.008 0.003 8.887 0.003
0 – 53 0.058 0.035 2.861 0.091
0 - 50
156

Table 4: A summary of the genera, height, and dbh of trees that parakeets nested in over
the course of 2001-2003. Some trees had more than one nest.
Genus Common Name n Height (m) dbh (cm)
Acer Maple 8 20.2 ± 5.6 58.5 ± 18.1
Cedrus Cedar 1 20.1 152.0
Fagus Beech 1 23.6 92.3
Fraxinus Ash 23 15.1 ± 7.9 48.5 ± 13.2
Juglans Walnut 1 15.9 90.1
Platanus Sycamore 4 35.2 ± 3.5 148.1 ± 24.7
Populus Poplar 6 20.5 ± 10.1 66.6 ± 31.8
Quercus Oak 15 20.7 ± 5.5 98.3 ± 24.8
Robinia Locust 1 18.5 95.5
Salix Willow 2 18.7 ± 1.8 79.6 ± 8.1
Tilia Lime 5 26.5 ± 3.7 71.7 ± 5.9
Ulnus Elm 2 12.4 ± 1.1 43.5 ± 4.7
Total 69 19.5 ± 8.1 73.7 ± 34.1
157

Table 5: A summary of the numbers of trees used by Rose-ringed Parakeets compared
with the numbers of randomly chosen trees.
Genus Common Name Number used Number of random
Acer Maple 8 (11.6%) 17 (37.0%)
Aesculus Horsechestnut 0 (0.0%) 4 (8.7%)
Alnus Alder 0 (0.0%) 1 (2.2%)
Betula Birch 0 (0.0%) 3 (6.5%)
Castanea Chestnut 0 (0.0%) 2 (4.3%)
Cedrus Cedar 1 (1.4%) 0 (0.0%)
Crataegus Hawthorn 0 (0.0%) 1 (2.2%)
Fagus Beech 1 (1.4%) 1 (2.2%)
Fraxinus Ash 23 (33.3%) 2 (4.3%)
Juglans Walnut 1 (1.4%) 0 (0.0%)
Platanus Sycamore 4 (5.8%) 0 (0.0%)
Populus Poplar 6 (8.7%) 1 (2.2%)
Quercus Oak 15 (21.7%) 9 (19.6%)
Robinia Locust 1 (1.4%) 0 (0.0%)
Salix Willow 2 (2.9%) 0 (0.0%)
Taxus Yew 0 (0.0%) 2 (4.3%)
Tilia Lime 5 (7.2%) 1 (2.2%)
Ulnus Elm 2 (2.9%) 2 (4.3%)
Total 69 (100%) 46 (100%)
158

Table 6: A summary of breeding success during 2001-2003, presented as mean ± s.e.
Overall, reproductive success averaged 1.9 ± 0.1 chicks fledged per nest.
Overall productivity 1.7 ± 0.2
2001 2002 2003 Total
(n = 33)
Isle of Thanet 1.7 ± 0.2
(n = 10)
SE London 2.1 ± 0.4
(n = 11)
Studland 1.5 ± 0.5
(n = 2)
SW London 1.4 ± 0.4
(n = 10)
2.0 ± 0.2
(n = 37)
2.2 ± 0.4
(n = 14)
1.9 ± 0.0.4
(n = 13)
1.8 ± 0.3
(n = 10)
1.9 ± 0.2
(n = 36)
1.8 ± 0.3
(n = 20)
2.0 ± 0.6
(n = 10)
1.9 ± 0.1
(n = 108)
1.9 ± 0.2
(n = 44)
2.0 ± 0.2
(n = 34)
- - 1.5 ± 0.50
2.0 ± 0.4
(n = 6)
(n = 2)
1.7 ± 0.2
(n = 26)
159

Table 7: A significant GLM model (n = 77, R 2 = 0.216, p < 0.001) for predicting
fledging success based on clutch size and the number of years the nest cavity was
occupied 1 .
Source β Std. Error Sum of
Squares
df Mean
Square
F Sig.
Corrected Model 31.959 2 15.979 10.208

Table 8: A significant GLM model (n = 77, R 2 = 0.107, p = 0.003) for predicting clutch
size based upon tree height 1 .
Source B Std. Error Sum of Squares df Mean Square F Sig.
Corrected Model 12.434 1 12.434 9.929 0.004
Intercept 4.596 0.323 279.567 1 279.567 203.038

Table 9: A comparison of fledging rates between three regions (SW London, SE London
and Coastal Kent) with those values reported by Pithon and Dytham (1999a)
SW London 1.2
Pithon and Dytham (1999a) This study
(n = 6)
SE London 0.25
(n = 4)
1.7 ± 0.2
(n = 26)
2.0 ± 0.2
(n = 34)
Isle of Thanet - 1.9 ± 0.2
(n = 44)
162

FIGURES:
Figure 1: Location of nests in the Greater London area. A total of 24 nests were located in
SW London and 34 nests in SE London over the course of the study.
163

Figure 2: Location of nests at the Isle of Thanet. A total of 46 nests were located at the
Isle of Thanet over the course of this study.
164

Figure 3: A summary of the percentage of nests that were reused from the previous year.
The highest rate of reuse was at the Isle of Thanet (n = 35), with fewer nests being reused
in SE London (n = 9) and SW London (n = 6).
Percentage of renests
90.0%
80.0%
70.0%
60.0%
50.0%
40.0%
30.0%
20.0%
10.0%
0.0%
SW London SE London
Region
Isle of Thanet
2002
2003
165

Figure 4: A histogram of first egg dates. These dates were calculated based on the date of
fledging. The median date of first egg is 23 March, but eggs may be laid from 27
February to 12 May (n = 108).
30
20
10
0
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
166

Figure 5: A histogram of clutch size. Clutch size averaged 3.7 ± 1.2 eggs (n = 77).
30
20
10
0
1
2
3
4
5
6
7
167

Figure 6: A histogram of the number of chicks fledged. The fledging rate averaged 1.9 ±
0.1 young per nest (n = 108).
40
30
20
10
0
0
1
2
3
4
5
168

Figure 7: A chart showing that the number of young fledged was higher when parakeets
reused a nest cavity (n = 35).
Number of young fledged ± SE
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
1 2
Number of years cavity occupied
3
169

Figure 8: The percentage of nests that were reused in successive years relative to the
number of young fledged (n = 35). Although it has been hypothesized that birds whose
nests failed should be less likely to reuse that nest in successive years, this was not the
case with the Rose-ringed Parakeets.
Percentage reuse
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 1 2
Number fledged
3 4
170

Appendix 1: A summary of nest location, clutch size, number of hatchlings, and fledging
success.
Location Latitude / Longitude Eggs Hatchlings Fledged
Ramsgate, Kent 51.342196 N x 1.434191 E 5 4 4
Ramsgate, Kent 51.342196 N x 1.434191 E 2
Ramsgate, Kent 51.342196 N x 1.434191 E 5 0 0
Ramsgate, Kent 51.342196 N x 1.434191 E 2 0 0
Ramsgate, Kent 51.342196 N x 1.434191 E 5 3 3
Ramsgate, Kent 51.342196 N x 1.434191 E 3 2 2
Margate, Kent 51.382084 N x 1.419038 E 5 4 4
Margate, Kent 51.382084 N x 1.419038 E 4 0 0
Margate, Kent 51.382084 N x 1.419038 E 3 1 1
Margate, Kent 51.375514 N x 1.369538 E 4 1 1
Beckenham, Kent 51.417710 N x 0.010640 W 4
Beckenham, Kent 51.417710 N x 0.010640 W 3 1 1
Beckenham, Kent 51.417710 N x 0.010640 W 4 3 3
Sidcup, Kent 51.428423 N x 0.127970 E 3
Sidcup, Kent 51.428423 N x 0.127970 E 1
Sidcup, Kent 51.428423 N x 0.127970 E 2 1 1
Sidcup, Kent 51.428423 N x 0.127970 E 2 0 0
Sidcup, Kent 51.428423 N x 0.127970 E 4 2 2
Sidcup, Kent 51.428423 N x 0.127970 E 2
Sidcup, Kent 51.428423 N x 0.127970 E 4 3 3
Bromley, Kent 51.382494 N x 0.052459 E 3
Studland, Dorset 50.643958 N x 1.948963 W 2 2 2
Studland, Dorset 50.643958 N x 1.948963 W 1 1 1
East Molesey, Surrey 51.404386 N X 0.350848 W 3 3 1
Harrow, Middlesex 51.583717 N x 0.301495W 4 3 3
Ham, Surrey 51.442515 N x 0.309314 W 3 0 0
Ham, Surrey 51.442515 N x 0.309314 W 3 3 3
Ham, Surrey 51.442515 N x 0.309314 W 6 0 0
Ham, Surrey 51.442515 N x 0.309314 W 4 3 3
Ham, Surrey 51.442515 N x 0.309314 W 2 2
Laleham, Surrey 51.404055 N x 0.487688 W 3 2 1
Thames Ditton, Surrey 51.398494 N x 0.336742 W 1
Thames Ditton, Surrey 51.398494 N x 0.336742 W 3 2 0
Margate, Kent 51.385272 N x 1.388297 E 5 0 0
Margate, Kent 51.385272 N x 1.388297 E 4 2 2
Margate, Kent 51.385272 N x 1.388297 E 2
Ramsgate, Kent 51.337302 N x 1.408378 E 2
Ramsgate, Kent 51.342196 N x 1.434191 E 2 2 2
171

Location Latitude / Longitude Eggs Hatchlings Fledged
Ramsgate, Kent 51.342196 N x 1.434191 E 3 1 1
Ramsgate, Kent 51.342196 N x 1.434191 E 3 1 0
Ramsgate, Kent 51.342196 N x 1.434191 E 2
Ramsgate, Kent 51.342196 N x 1.434191 E 3 3 3
Ramsgate, Kent 51.342196 N x 1.434191 E 4
Ramsgate, Kent 51.342196 N x 1.434191 E 2
Ramsgate, Kent 51.342196 N x 1.434191 E 2
Margate, Kent 51.382084 N x 1.419038 E 4 4 4
Margate, Kent 51.375514 N x 1.369538 E 5 5 5
Beckenham, Kent 51.417710 N x 0.010640 W 5 1 1
Beckenham, Kent 51.417710 N x 0.010640 W 2 0 0
Beckenham, Kent 51.417710 N x 0.010640 W 4 2 2
Beckenham, Kent 51.417710 N x 0.010640 W 2
Sidcup, Kent 51.428423 N x 0.127970 E 3 3 3
Sidcup, Kent 51.428423 N x 0.127970 E 3 2 2
Sidcup, Kent 51.428423 N x 0.127970 E 2 0 0
Sidcup, Kent 51.428423 N x 0.127970 E 5 5 5
Sidcup, Kent 51.428423 N x 0.127970 E 1
Sidcup, Kent 51.428423 N x 0.127970 E 2
Sidcup, Kent 51.428423 N x 0.127970 E 3
Bromley, Kent 51.382494 N x 0.052459 E 1
Bromley, Kent 51.382494 N x 0.052459 E 3
East Molesey, Surrey 51.404386 N X 0.350848 W 4 2 2
Ham, Surrey 51.442515 N x 0.309314 W 5 3 3
Ham, Surrey 51.442515 N x 0.309314 W 3 0 0
Ham, Surrey 51.442515 N x 0.309314 W 1
Ham, Surrey 51.442515 N x 0.309314 W 2
Ham, Surrey 51.442515 N x 0.309314 W 3
Ham, Surrey 51.442515 N x 0.309314 W 1
Laleham, Surrey 51.404055 N x 0.487688 W 4 1 1
Laleham, Surrey 51.404055 N x 0.487688 W 4 2 2
Laleham, Surrey 51.404055 N x 0.487688 W 3
Margate, Kent 51.385272 N x 1.388297 E 1
Ramsgate, Kent 51.337302 N x 1.408378 E 2
Ramsgate, Kent 51.342196 N x 1.434191 E 6 2 2
Ramsgate, Kent 51.342196 N x 1.434191 E 4 4 4
Ramsgate, Kent 51.342196 N x 1.434191 E 3 1 0
Ramsgate, Kent 51.342196 N x 1.434191 E 2 1 1
Ramsgate, Kent 51.342196 N x 1.434191 E 4 3 3
Ramsgate, Kent 51.342196 N x 1.434191 E 3 3 3
Ramsgate, Kent 51.342196 N x 1.434191 E 5 0 0
Ramsgate, Kent 51.342196 N x 1.434191 E 7 0 0
Ramsgate, Kent 51.342196 N x 1.434191 E 3 2 2
172

Location Latitude / Longitude Eggs Hatchlings Fledged
Ramsgate, Kent 51.342196 N x 1.434191 E 4 2 2
Ramsgate, Kent 51.342196 N x 1.434191 E 5 2 2
Ramsgate, Kent 51.342196 N x 1.434191 E 6 4 3
Ramsgate, Kent 51.342196 N x 1.434191 E 4 2 2
Broadstairs, Kent 51.368484 N x 1.428874 E 3 1 1
Northdown, Kent 51.382084 N x 1.419038 E 5 2 1
Northdown, Kent 51.382084 N x 1.419038 E 1
Northdown, Kent 51.382084 N x 1.419038 E 4 2 2
Northdown, Kent 51.382084 N x 1.419038 E 2 0 0
Northdown, Kent 51.382084 N x 1.419038 E 6 3 3
Northdown, Kent 51.382084 N x 1.419038 E 4 3 3
Beckenham, Kent 51.417710 N x 0.010640 W 5 5 3
Beckenham, Kent 51.417710 N x 0.010640 W 5 2 0
South Norwood, Kent 51.402967 N x 0.089307 W 1 0 0
Sidcup, Kent 51.428423 N x 0.127970 E 3 3 3
Sidcup, Kent 51.428423 N x 0.127970 E 3 1 1
Sidcup, Kent 51.428423 N x 0.127970 E 4
Sidcup, Kent 51.428423 N x 0.127970 E 5 4 4
Sidcup, Kent 51.428423 N x 0.127970 E 4 4 4
Sidcup, Kent 51.428423 N x 0.127970 E 2 1 1
Sidcup, Kent 51.428423 N x 0.127970 E 3 0 0
Twickenham, Middlesex 51.445019 N x 0.380280 W 4 1 1
Twickenham, Middlesex 51.445019 N x 0.380280 W 1
Twickenham, Middlesex 51.445019 N x 0.380280 W 3 2 2
Ham, Surrey 51.442515 N x 0.309314 W 2
Laleham, Surrey 51.404055 N x 0.487688 W 5 3 3
Laleham, Surrey 51.404055 N x 0.487688 W 3 3 3
173

Chapter 5:
Aging & sexing parakeets
ABSTRACT:
Although adult male Rose-ringed Parakeets Psittacula krameri (those > 3 years
old) can easily be identified due to their rose-coloured ring and black bib, sexually
immature males (those < 3 years old) and females are considered impossible to separate
in the hand. However, while the biometrics of males and females do broadly overlap,
males tend to be slightly larger than females in all measurements. I tested whether a
discriminant function could be derived to separate the two sexes using multiple
measurements. A total of 192 parakeets (+ 28 chicks) were captured and measured from
17 February 2001 to 29 May 2003. Measurements of wing length, tail length, bill length,
toe length, mass, and number of yellow underwing coverts were recorded for these
individuals. In addition, photographs of the head, upper wing, and lower wing were also
taken. Volunteers ringed an additional 43 birds and recorded measurements on wing
length, tail length, and mass (bringing the total ringed to 263). Feather and blood samples
were taken from 45 (22 females and 23 males) of these birds and subjected to
haplotyping. The measurements from these known-sex individuals were pooled with the
measurements for adult males and a binary logistic regression function that correctly
separated 96.6% of the known sexes was derived. The first data on moult patterns in the
UK were obtained and are useful in aging birds during the spring. In addition, it was
174

found that the tips of primaries were rounder in adult birds than in juveniles, a result that
agrees with other studies on parrots and other bird species worldwide.
175

INTRODUCTION:
As of the end of 2000, a total of 42 Rose-ringed Parakeets Psittacula krameri
were ringed in the UK (Clark et al. 2002). However, accurate identification of age and
sex classes of Rose-ringed Parakeets in Britain is a problem. It is straightforward to
identify adult male Rose-ringed Parakeets, as they have a bright rose-coloured ring
around their neck, as well as a black bib. Separating sub-adult males from females
however is far more difficult, as these individuals are uniformly green and lack any
obvious distinguishing characteristics.
Several methods have been employed in order to sex monomorphic species. For
instance, behaviour has often been utilized to assign sexes in a pair of breeding birds (Fox
et al. 1981). However, this technique is only useful for a short time during the year (the
breeding season) and is not suitable for assigning sexes to non-breeding birds (Coulson et
al. 1983). Vent measurements have also been suggested as a method for sexing birds, but
again this technique is only suitable for pairs during the breeding season for a short
period after the female has laid her eggs (Boersma & Davies 1987). Laparotomy, in
contrast, can be used to sex individuals throughout the year (Maron & Myers 1984).
However, laparotomy is an intrusive and time-consuming technique and may be risky to
the birds as well (Coulson et al. 1983). Sexing birds by HAPLOTYPING is less intrusive
than laparotomy, as it requires only a sample of DNA, typically either from a blood
sample or from a feather sample (de Mattos et al. 1998, Griffiths et al. 1998, Murata et al.
1998, Baker et al. 1999), but it requires time in the laboratory in order to obtain the
results and so is not suitable for sexing birds in the field.
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One possible way to quickly sex these individuals in the field is to use biometrics.
A number of authors have succeeded in identifying the sexes of sexually monomorphic
species by means of discriminant functions (Fox et al. 1981, Scolaro et al. 1983, Hanners
& Patton 1985, Granadeiro 1993, Baker et al. 1999). According to published accounts,
male and female Rose-ringed Parakeets have different wing lengths, tail lengths, toe
lengths, and bill lengths (although there is some overlap) and so may be suitable for
discriminant function sexing. Table 1 summarizes these differences.
In theory, it should be relatively straightforward to separate males from females
using biometrics, provided that the subspecies has been identified. Unfortunately, it
appears that the Rose-ringed Parakeets in the UK are a mixture of two subspecies; P. k.
manillensis and P. k. borealis (Morgan 1993, Pithon & Dytham 2001). This mixture of
subspecies may render invalid the use of biometrics to determine sex, as the
measurements for female P. k. borealis broadly overlap with the measurements for male
P. k. manillensis.
Aging parakeets also presents a challenge, as little information has been
published. A few authors have noted that patterns of flight feather moult and wear are
useful in aging parrots and parakeets (Bucher et al. 1987, Emison et al. 1994, Mawson &
Massam 1996), and it is possible that these patterns may be mirrored in Rose-ringed
Parakeets. In addition, juvenile parrots tend to have more pointed primaries than adult
parrots (Emison et al. 1994).
When aging Rose-ringed Parakeets, only one fact is known for certain: adult
males have a rose-coloured ring and a black bib that develops by their third year
(Forshaw 1989, Juniper & Parr 1998). A number of suggestions have been made for
177

differentiating adult birds from juveniles. For instance, it has been suggested that
juveniles have a yellowish fringe on their feathers (Roselaar 1985). They are also
reported to have greyish-white irises; adult birds are reported to have yellowish irises
(Forshaw 1989). Finally, some juvenile flight feathers may be present until the third year
(Roselaar 1985). However, none of these suggestions or reports has been examined
thoroughly.
The population of Rose-ringed Parakeets has increased exponentially during the
late 1990s and early 21 st century (Butler 2002), and so it is likely that an increasing
number will be ringed in the near future. If it were possible to accurately age and sex
Rose-ringed Parakeets, information on age structure and sex ratios in the UK population
could be gathered and used to model the populations. Here I show that it is indeed
possible to age and sex parakeets in the UK using a discriminant function derived from
biometrics combined with flight feather moult patterns.
METHODS:
From 17 February 2001 to 29 May 2003, Rose-ringed Parakeets were caught with
mist nets and measured at fourteen sites (eleven in SW London, two in SE London, and
one at the Isle of Thanet). Nearly all of these birds were ringed at feeders, although a few
were ringed as chicks and a single bird was caught at the roost in Esher. Measurements
were taken on flattened wing length, tail length, toe length (not including the claw), bill
length (bill tip to skull), number of yellow underwing coverts (see Figure 1), and mass. A
stopped wing rule (readable to 1 mm) was used to measure wing length and tail length,
while Vernier callipers (readable to 0.1 mm) were used to measure toe length and bill
178

length. Birds were weighed on a Pesola spring balance (readable to 1 g). In addition,
maximum and minimum tarsus measurements were recorded on 41 birds. Volunteers who
ringed additional birds shared their information on wing length, tail length, and mass.
The head, upper wing and lower wing were photographed in an effort to
determine if previously unnoticed plumage differences and/or eye colour might allow
separation of the sexes. Finally, feathers were collected from 45 birds lacking rose-
coloured neck rings (i.e. subadult males and females) and were subjected to haplotyping
(determination of the sex by DNA) in a procedure similar to that described elsewhere (de
Mattos et al. 1998, Murata et al. 1998) by Simon Griffith of the University of Oxford.
The measurements from these known sex individuals were then pooled with the
measurements on adult males and a logistic regression function was then derived.
The Natural History Museum at Tring was visited in order to examine the
specimens for signs of moult. Because British parakeets are derived from Indian birds,
only parakeets from the two Indian subspecies (P. k. manillensis and P. k. borealis) were
examined. A total of 168 individuals were examined; 108 individuals of P. k. borealis,
52 individuals of P. k. manillensis, 7 feral individuals, and 1 captive individual. Only 2
known females were included in the collection, while 106 males confirmed males were
included (including one subadult male that lacked the rose-coloured neck ring but had
enlarged testes). Data on moult condition and date were recorded.
All statistics were performed on SPSS 11.0 and results are presented as mean ±
standard deviation.
179

RESULTS:
A total of 235 adult parakeets (+28 chicks) were ringed over the course of the
study, with the author taking full measurements on 192 adult birds, and with volunteers
providing measurements for an additional 43 adult birds. The vast majority of the birds
were ringed in SW London (221 parakeets), with only a few ringed in SE London (9
parakeets) and the Isle of Thanet (5 parakeets). (See Figures 2 and 3 for a map of the
locations where parakeets were ringed).
Average values for both confirmed males and females, as well as for all the
parakeets measured, are presented in Table 2. As mentioned earlier, feral Rose-ringed
Parakeets in the UK are believed to have derived from a mixture of the two Indian
subspecies (Morgan 1993, Pithon & Dytham 2001). In general, however, the
measurements of these parakeets appeared more similar to those of P. k. borealis than P.
k. manillensis (see Tables 1 and 2). Wing lengths of known males, for example, averaged
178.1 ± 4.5 mm, and ranged from 169 to 189 mm (n = 72). Both the average and the
ranges agree well with the data provided for P. k. borealis in Roselaar et al. (1985),
Forshaw (1989), and Pithon & Dytham (1998) (see Table 1). However, the bill
colouration of most of the individuals captured consisted of a reddish upper mandible and
a dark lower mandible – a description that more closely matches P. k. manillensis.
A total of 94 individuals were identified to sex (45 by haplotyping, while the
remaining 49 were identified as adult males by the presence of a pink ring around their
collar). A binary logistic function was created to predict sex based on wing length, tail
length, mass, bill length, toe length and the number of completely yellow underwing
coverts (n = 86; incomplete data were recorded on 8 birds). Backwards deletion of the
180

variables was undertaken in order to retain only those elements that are at least weakly
significant (defined as p < 0.1). This generated a significant binary logistic regression
function (χ 2 = 85.079, d.f. = 3, Nagelkerke R 2 = 0.918, p < 0.001), which relies upon only
wing length, bill length and the number of completely yellow underwing coverts:
f(x) = (0.486*wing length in mm) + (2.143 * bill length in mm) - (2.448 * the number of
yellow underwing coverts) - 131.958.
Again, utilizing this function results in a positive value for males and a negative value for
females. This logistic function was accurate in placing 96.6% of the sexes into their
appropriate categories (n = 88; incomplete data were recorded for 6 of the birds, see
Table 3). All elements of this function are at least weakly significant (see Table 4).
All known-sex birds with wing lengths greater than 174 mm were males, while all
known-sex birds with wing lengths less than 169 mm were females (Table 2). However,
44.3% of the parakeets captured had wing lengths that fell between 169 and 174 mm. All
known-sex birds with bill lengths greater than 26.3 mm were males (Table 2) but only
23.9% of the birds captured had bill lengths greater than 26.3 mm. All known-sex birds
with zero yellow underwing coverts were males, while all known-sex birds with 4 or
more yellow underwing coverts were females (Table 2) but 46.8% of the parakeets
caught had an intermediate amount of yellow (e.g. 1-3 completely yellow underwing
coverts)..
Aging the birds proved to be more complicated due to the limited number of
known-age birds. The suggestion that iris colour could be used to help age birds did not
181

appear to be accurate for the parakeets in the UK population, as adult males (i.e. those
with a rose-coloured ring around their neck) showed irises that ranged from pale yellow
to pale grey to dark grey (Figure 4). Likewise, the yellowish edging on primary feathers
that supposedly indicates an immature bird can also be seen on the primary feathers of
adult males (Figure 5).
The shape of the primaries, however, did change as parakeet aged. Juvenile
parakeets (those that had recently fledged the nest) had very pointed primaries that can be
seen in Figure 6. Figure 7 shows a one year-old bird moulting its juvenile flight feathers.
The new feathers are noticeably broader at the tips. Figure 8 shows a two-year old male
parakeet with very broad primaries and the tips of the primaries of an adult male parakeet
(e.g. one 3+ years old) are likewise very broad (see Figure 9). From these photographs, it
is evident it is possible to identify juvenile birds until they are approximately one year old
based on the relative narrowness of their primary tips. Thereafter, the tips of the primary
feathers are fairly broad, and separating a one year-old parakeet from a two or three year-
old parakeet may not be possible.
Over the course of this study, the first information on moult in British Rose-
ringed Parakeets was amassed. According to the literature, male Rose-ringed Parakeets
take three years to develop their rose-coloured neck ring, but the time of year that they
develop their adult plumage does not appear to have been documented. On 8 March 2002,
a bird was caught that had a few blue-tipped feathers on its head, but no pink ring on its
neck. On 22 March 2002, a bird was seen (but not caught) with a partial neck ring. On 4
April 2002, a single parakeet was caught with some blue feathers on its head, but lacked
the neck ring. Additionally, one bird was caught on 17 July 2001 that had a single rose-
182

coloured feather present on its neck. It appears therefore, that the characteristic male
adult plumage develops between March and July in the UK.
During the course of this study, 21 birds were captured in various stages of
primary moult from 8 May to 17 July (see Table 5). Primary moult begins in feathers 5-6
and then spreads out in both directions, as expected. None of the 21 birds captured while
moulting exhibited a suspended moult.
An examination of the specimens at Tring revealed that primary moult varied
between subspecies. P. k. borealis was recorded moulting from 1 August until 24
September (n = 10) with a single bird in primary moult on 28 December. In contrast, P. k.
manillensis was recorded moulting from 3 May to 22 May (n = 2) and again from 5 July
to 7 August (n = 8). A single bird was moulting in September (exact date unknown) and
two birds were moulting on 6 December. Moult strategies of UK birds, therefore, appear
more similar to P. k. manillensis than to P. k. borealis.
Finally, the first information on maximum and minimum tarsus diameter in this
species in the UK was collected. Minimum tarsus measurement was 4.9 ± 0.3 and
maximum tarsus diameter was 5.4 ± 0.2 (n = 41; see Table 6).
DISCUSSION:
The number of parakeets ringed over the course of this study (n = 245) is far
greater than the 42 parakeets that had previously been ringed up until 2000 (Clark et al.
2002). The greatest numbers of parakeets were ringed in the SW London area, as
approximately 75% of the parakeets in the UK reside in this general area (Butler 2002).
The populations in SE London and the Isle of Thanet are considerably smaller.
183

Consequently, fewer gardens have parakeets coming to bird feeders which limits the
number of locations where ringing can occur. Ringing of the chicks tends to be very
difficult, as most parakeets nest in Green Woodpecker cavities that are too small for the
author or his field assistant to fit his or her hand into.
Catching parakeets as they come to feeders may potentially introduce several
biases. For instance, it is conceivable that parakeets may avoid returning to the feeders
after being captured, or they may avoid flying into the mist nets when coming to the
feeders in the future. In addition, mist netting at feeders may introduce biases into in the
age or sex structure of the samples, as certain ages and/or sexes may be more likely to use
feeders.
While immature male and adult female parakeets are superficially similar, it is
possible to separate the sexes using a binary logistic regression based on wing length, bill
length, and the number of completely yellow greater underwing coverts. This formula
accurately placed 96.6% of all known sexes, but should still be treated with caution due
to the relatively low number of known females (n = 22).
Parakeets in the UK tend to show characteristics of both P. k. borealis and P. k.
manillensis, a result that has been published elsewhere (Morgan 1993, Pithon and
Dytham 2001). However, on average the parakeets caught in the UK were heavier than
indicated by the published literature. Of particular interest was a parakeet caught on 8
March 2002, which weighed 180 g and was presumably a gravid female. This is nearly
14% heavier than the heaviest bird reported in the literature, a gravid female from Nepal
that weighed 158 g (Diesselhorst in Roselaar 1985).
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The discovery that juvenile Rose-ringed Parakeets have more pointed primaries
than adults agrees with our studies on flight feather shape in parrots (e.g. Emison et al.
1994). However, the study by Emison et al. (1994) found that successive flight feather
moults resulted in successively more rounded primaries. This may hold true for Rose-
ringed Parakeets as well, but it is very difficult to distinguish the relative degrees of
roundness of parakeets in the hand (with the exception of juveniles). Perhaps future work
involving measuring the tips of the primaries might succeed in quantifying such a subtle
feature.
To date, it is still not possible to age Rose-ringed Parakeets in the hand, with a
few exceptions. These exceptions are as follows:
1.) Birds with complete rose-coloured rings around their necks are adult males
(i.e. they were born 3+ years ago).
2.) Parakeets with pointed primaries are juveniles.
3.) Parakeets showing a few bluish-tinged feathers on the head and/or an
incomplete neck ring during the period March to July are male birds that were
probably born two years ago.
In addition, parakeets undergoing wing moult in May and early June are unlikely
to be breeding birds, as adult parakeets engage in post-breeding moult (Juniper and Parr
1998, Pithon and Dytham 2001).
In general, primary moult in parrots begins in the centre of the wing and then
proceeds both toward the body and toward the wing tip (Juniper & Parr 1998). Parrots
185

undergo a complete moult on an annual basis, with adults typically engaging in a post-
breeding moult (Juniper & Parr 1998). Juvenile birds typically replace their feathers
before they are one year old (Juniper & Parr 1998). Although it has been reported that
some Rose-ringed Parakeets may retain juvenile flight feathers into their third year
(Roselaar 1985), I did not see any evidence of juvenile flight feathers in adult birds, or in
birds known to be more than a year old.
Roselaar (1985) states that moult may continue through November in European
birds. Although no primary moult was observed after July, it is certainly possible that
they continue moulting until August and possibly into September. Indeed, after the
completion of this study, moulting parakeets were caught in August and September by
the Runnymede Ringing Group (Ross, pers. comm..)
According to Roselaar (1985), primary moult takes more than one year, and is
suspended at the end of the season. At the beginning of the next moulting season, the
moult continues from the point where it left off. Once the remaining old feathers have
been moulted, the moult begins again from p6. However, none of the 21 birds that we
captured while moulting exhibited a suspended moult (i.e. none of the birds early in the
moulting season were moulting any feathers other than p5 or p6).
The timing of primary moult in UK parakeets appears to be more similar to that of
P. k. manillensis than P. k. borealis. Moult in P. k. manillensis was recorded in May
(presumably non-breeding individuals) and again in July and August (presumably post-
breeding individuals). The lack of moulting birds in the UK during August may be due
simply to the low number of moulting birds captured during this study.
186

It would be worthwhile for a future study to examine ultraviolet reflectance in this
species. It has recently been demonstrated that mate selection in Budgerigars
(Melopsittacus undulatus) is influenced by ultraviolet reflectance (Pearn et al. 2001) and
it is possible that ultraviolet reflectance between subadult males and females may vary.
Finally, a word of caution for future studies that involve ringing Rose-ringed
Parakeets. The BTO (British Trust for Ornithology) currently recommends that these
parakeets be fitted with a ring that has an internal diameter of 5.25 mm (a size “D2” ring;
Redfern and Clark 2002). However, the tarsus on this species is larger than the BTO
credits. Maximum (5.4 ± 0.2) and minimum (4.9 ± 0.3) tarsus measurements (n = 41)
taken during the course of this study indicate that a larger ring should be used. The next
larger size ring has an internal diameter of 7.0 mm (a size “E” ring) and is more
appropriate for this species.
187

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191

TABLES:
Table 1: Wing, tail, bill, and toe measurements for the four subspecies of Psittacula
krameri based on three published sources (Forshaw 1989, Pithon 1998, Roselaar 1985).
An asterisk (*) denotes missing data.
Wing (male)
in mm
Wing (female)
in mm
P. k.
krameri
P.k.
parvirostris
P. k.
borealis
P.k.
manillensis
Author
150 (144-157) * 178 (172-187) 165 (160-169) Cramp
150 (144-157) 153 (146-160 174 (170-177) 170 (162-180) Forshaw
152 (150-154) 155 (149-160) 177 (176-178) 163 (160-165) Pithon 1
148 (143-152) * 173 (168-178) 158 (154-160) Cramp
148 (143-152) 153 (146-160) 172 (170-175) 163 (153-167) Forshaw
146 (144-148) 152 (124-180) 169 (167-171) 157 (150-163) Pithon 1
Tail (male) in 231 (194-278) * 253 (229-279) 205 (182-235) Cramp
mm 231 (194-278) 234 (215-246) 239 (226-253) 219 (203-235) Forshaw
Tail (female) 198 (177-240) * 221 (204-238) 178 (164-188) Cramp
in mm 198 (177-240) 196 (184-218) 220 (211-230) 193 (174-210) Forshaw
Bill (male) in
mm
Bill (female) in
mm
19.6 (18-21) * 23.8 (23.2- 24.2 (23.1- Cramp
26.4)
25.4)
19.6 (18-21) 19.6 (19-21) 23.2 (22-25) 23.3 (22-25) Forshaw
22.0 (21.8- 22.0 (21.0- 27.0 (26.6- 25.8 (25.0- Pithon
22.2)
22.9)
27.4)
26.4)
1
19.8 (18-21 * 22.6 (20.8- 22.5 (22.2- Cramp
24.4)
22.8)
19.8 (18-21) 19.6 (19-21) 23.0 (21-24) 22.6 (21-24) Forshaw
22.0 (21.8- 22.0 (21.0- 27.0 (26.6- 25.8 (25.0- Pithon
22.2)
22.9)
27.4)
26.4)
1
* * 28.6 (26.9-
Toe (male) in
30.7)
mm 9.2 (9.0-9.4) 9.1 (8.5-9.7) 10.2 (10.0-
10.4)
* * 28.1 (26.2-
Toe (female)
29.3)
in mm 9.2 (9.0-9.4) 9.1 (8.5-9.7) 10.2 (10.0-
10.4)
* Cramp
9.3 (9.0 - 9.4) Pithon 1
* Cramp
9.3 (9.0-9.4) Pithon 1
1 = Pithon reported 95% C. I. rather than the extremes reported by the other two authors
192

Table 2: A summary of the biometrics for parakeets trapped in the UK during this study.
Results are presented as mean ± standard deviation, with extremes noted in parentheses.
Wing
(mm)
All 173.7 ±
Known
males
Known
females
5.2
(159.0-
189.0)
n = 241
178.1 ±
4.5
(169.0-
189.0)
n = 72
168.9 ±
3.3
(162.0-
174.0)
n = 22
Tail
(mm)
207.0 ±
28.9
(140.0-
284.0)
n = 233
225.8 ±
29.4
(157.0-
284.0)
n = 72
188.3 ±
31.6
(142.0-
242.0)
n = 22
Mass (g) Bill
141.4 ± 9.8
(116.0-
180.0)
n = 238
142.2 ± 9.0
(124.0-
166.0)
n = 71
140.4 ±
12.1
(124.0 -
180)
n = 22
(mm)
25.4 ±
1.1
(20.8-
27.7)
n = 197
26.1 ±
0.9
(23.0-
27.6)
n = 67
25.1 ±
0.6
(24.0-
26.3)
n = 22
Toe
(mm)
24.6 ±
1.3
(20.7-
27.9)
n = 195
24.8 ±
1.1
(22.6-
27.9)
n = 65
24.3 ±
1.0
(22.0-
26.0)
n = 22
No. of
yellow
underwing
coverts
2.0 ± 1.9
(0-8)
n = 141
0.7 ± 0.9
(0-3)
n = 66
3.8 ± 1.4
(1-6)
n = 22
193

Table 3: Classification table for a binary logistic regression function that uses only wing
length, bill length, and the number of completely yellow underwing coverts. Of the 88
birds, a total of 85 (96.6%) were classified correctly.
Observed
Predicted
Male Female Percentage
Correct
Male 64 2 97.0%
Female 1 21 95.5%
Overall: 96.6%
194

Table 4: Logistic regression predicting gender based on wing length, bill length, and the
number of completely yellow underwing coverts.
Predictor B Wald χ2 p-value Odds Ratio
Wing 0.486 2.908 p = 0.088 0.615
Bill 2.143 3.544 p = 0.060 0.117
Yellow -2.448 6.132 p = 0.013 11.563
Constant -131.958 5.624 p = 0.018 2.04 * 10 57
195

Table 5: Moult scores for primaries 1-10 on 21 birds that were moulting when captured.
Moult scores range from 0 (old feather) to 5 (new, fully grown feather).
1 2 3 4 5 6 7 8 9 10
8 May 0 0 0 0 0 4 0 0 0 0
8 May 0 0 0 0 0 3 0 0 0 0
8 May 0 0 0 0 0 4 0 0 0 0
9 May 0 0 0 0 4 0 0 0 0 0
9 May 0 0 0 0 4 0 0 0 0 0
9 May 0 0 0 0 0 3 0 0 0 0
19 May 0 0 0 0 0 3 0 0 0 0
19 May 0 0 0 0 2 0 0 0 0 0
19 May 0 0 0 0 0 2 0 0 0 0
20 May 0 0 0 0 0 1 0 0 0 0
20 May 0 0 0 0 0 3 0 0 0 0
24 May 0 0 0 0 4 0 0 0 0 0
24 May 0 0 0 0 2 0 0 0 0 0
30 May 0 0 0 0 3 0 0 0 0 0
30 May 0 0 0 0 5 5 3 0 0 0
6 June 0 0 0 0 1 0 0 0 0 0
9 June 0 0 0 0 4 5 1 0 0 0
24 June 0 0 0 0 0 3 0 0 0 0
24 June 0 0 0 2 5 5 2 0 0 0
17 July 0 0 0 3 5 5 0 0 0 0
17 July 0 0 0 0 0 1 0 0 0 0
196

Table 6: Date, ring number, and tarsus measurements (in mm) for 41 parakeets.
Date Ring No. Min tarsus (mm) Max tarsus (mm)
18 Feb 2002 EG50366 4.5 5.6
18 Feb 2002 EG50367 4.2 5.4
18 Feb 2002 EG50368 5.0 5.2
18 Feb 2002 EG50369 4.7 5.5
18 Feb 2002 EG50371 4.9 5.6
1 Mar 2002 EG50373 4.5 5.3
8 Mar 2002 EG50375 5.0 5.6
8 Mar 2002 EG50376 4.9 5.5
8 Mar 2002 EG50377 4.4 5.3
8 Mar 2002 EG50378 5.3 5.8
8 Mar 2002 EG50379 4.6 5.4
12 Mar 2002 EG50380 5.1 5.4
22 Mar 2002 EG50384 4.6 5.6
22 Mar 2002 EG50385 5.0 5.4
22 Mar 2002 EG50386 4.6 5.4
22 Mar 2002 EG50387 4.5 5.2
23 Mar 2002 EG50388 5.0 5.3
23 Mar 2002 EG50389 5.4 5.6
23 Mar 2002 EG50390 5.0 5.4
24 Mar 2002 EG50391 5.2 5.5
2 Apr 2002 EG50392 5.2 5.6
2 Apr 2002 EG50393 5.1 5.3
2 Apr 2002 EG50394 5.0 5.4
4 Apr 2002 EG50395 4.9 5.3
4 Apr 2002 EG50396 5.0 5.7
4 Apr 2002 EG50397 5.1 5.4
4 Apr 2002 EG50398 5.4 5.6
4 Apr 2002 EG50399 5.0 5.2
4 Apr 2002 EG50400 5.2 5.5
21 Apr 2002 EG51251 4.7 5.5
22 Apr 2002 EG51252 4.7 5.2
22 Apr 2002 EG51253 4.8 5.3
22 Apr 2002 EG51254 5.2 5.9
22 Apr 2002 EG51255 4.6 5.3
22 Apr 2002 EG51256 5.0 5.7
22 Apr 2002 EG51257 4.8 5.7
22 Apr 2002 EG51258 4.7 5.6
22 Apr 2002 EG51259 5.0 5.3
22 Apr 2002 EG51260 5.3 5.4
22 Apr 2002 EG51261 4.4 5.2
23 Apr 2002 EG51262 4.9 5.4
23 Apr 2002 EG51264 5.2 5.6
Mean ±
SD
4.9 ± 0.3 5.4 ± 0.2
197

FIGURES:
Figure 1: A comparison of the number of yellow underwing coverts. The adult male (top
photo) has no completely yellow underwing coverts. In contrast, the female (lower photo)
has six completely yellow underwing coverts
198

Figure 2: Locations in the Greater London area where Rose-ringed Parakeets were
ringed. The city of London is provided as a reference point. A total of 192 parakeets were
ringed at 11 locations.
199

Figure 3: Locations at the Isle of Thanet, Kent, where Rose-ringed Parakeets were ringed.
The city of Margate is provided as a reference point. A total of 20 parakeets were ringed
at three locations.
200

Figure 4: Photographs of iris colour in adult males. Although juvenile birds are supposed
to have grayish-white irises and adult birds are supposed to have yellowish irises
(Forshaw 1989), iris colour in adult male parakeets in the UK ranged from yellowish-
grey (left photo) to pale grey (middle photo) to dark grey (right photo).
201

Figure 5: The yellowish edging on primary feathers that supposedly indicates an
immature bird (Roselaar 1985) can often be seen on adult male birds. The yellowish
primary edging can be seen in the top photo of a recently fledged parakeet as well as the
bottom photo of an adult male parakeet.
202

Figure 6: A photograph of a recently fledged parakeet showing the primaries and the
secondaries. Note how pointed the first four primaries are (p1-4). This parakeet
(EG51219) was caught on 21 July 2002.
203

Figure 7: A photograph of primaries and secondaries on a one-year old parakeet.
EG50336 was ringed as a chick in a nest in a garden in East Molesey on 19 May 2001.
She was recaptured in the same garden on 21 July 2002, when she was a year old. Note
that she is moulting primaries 4 and 8 (p4 and p8). Primaries p5-p7 are much broader at
the tips than P1-P3 and are presumably freshly moulted.
204

Figure 8: A photograph of primaries and secondaries on a two-year old male parakeet.
EG50377 was caught on 8 March 2002 and was just beginning to show some blue
feathers on his head, indicating that he is a male entering his third year.
205

Figure 9: A photograph of the primaries and secondaries of an adult male parakeet (e.g.
>3 yrs old). Note how rounded the primaries and secondaries are on EG51228,
particularly the first four primaries (p1-4).
206

Appendix 1: Measurements taken on known-sex birds. An asterisk (*) denotes a
missing value.
Ring Sex Tail Wing Mass Bill Toe Yellow
(mm) (mm) (g) (mm) (mm)
EG50328 F 183 171 152.7 25.9 24.1 2
EG50337 F 231 170 130 24 23 1
EG50339 F 242 174 145 25 26 5
EG50341 F 153 168 141 24.3 22 4
EG50344 F 225 174 137 25 25 3
EG50362 F 220 171 132.1 25 23.4 *
EG50363 F 167 167 131.4 24.7 23.5 3
EG50365 F 191 173 150 24.4 24.1 6
EG50355 F 164 165 138.4 25.4 23.6 5
EG50379 F 236 170 180 25.7 25.7 *
EG50311 M 224 180 166 25.3 25.2 0
EG50325 M 204 174 137.5 25 24.6 1
EG50326 M 211 182 151.6 25.2 26.1 0
EG50327 M 218 189 144.5 26.5 24.2 1
EG50340 M 240 188 152 27 24 1
EG50342 M 200 182 142 26 23 1
EG50343 M 196 174 136 23 23 0
EG50346 M 248 179 153.7 27 25 0
EG50347 M 220 183 138.7 27.2 23.3 1
EG50351 M 267 176 150 24.8 24.3 0
EG50358 M 232 174 142 26.5 23.9 1
EG50359 M 170 171 139 26.3 23 0
EG50360 M 245 179 132.2 24.6 24.9 0
EG50361 M 183 180 143.9 26 18.7 0
EG50364 M 187 173 144.2 26.6 25.1 3
EG50352 M 168 181 161 25.5 23.6 0
EG50354 M 246 185 148.8 25.8 24 1
EG50356 M 163 177 156.1 27.2 27.1 2
EG50375 M 213 174 139 27.6 25.6 0
EG50376 M 226 174 142 27.4 25.1 0
EG50378 M 243 180 149 27 25.6 1
EG50386 M 245 181 143 27.1 26.1 0
EG50387 M 208 180 140 25.8 24 1
EG50389 M 248 179 124 26.3 26.7 2
EG50391 M 211 175 129 26.4 24.5 1
EG50393 M 248 179 137 26.9 24.5 1
EG50394 M 263 181 162 27.1 25.7 0
EG50397 M 240 176 146 26.4 26.2 0
EG50398 M 157 177 142 26.0 26.4 *
207

Ring Sex Tail Wing Mass Bill Toe Yellow
(mm) (mm) (g) (mm) (mm)
EG50399 M 242 176 135 26.4 25 0
EG51253 M 172 180 139 26.8 23.3 1
EG51255 M 226 176 128 25.7 25.2 0
EG51259 M 226 172 132 25.5 23.1 1
EG51260 M 253 180 144 24.5 24.1 0
INJURED M 247 185 146 27.2 * *
EG51264 M 246 176 149 25.9 24.5 1
EG51268 M 234 178 151 25.8 26.4 1
EG51269 M 235 179 142 26.2 24 2
NORING M 261 187 135 26.2 25.4 1
EG51270 M 257 185 159 26.5 25.2 1
EG51271 M 263 178 146 25.9 24.7 1
EG51272 M 205 182 147 26.3 24.7 1
EG51273 M 246 178 138 27 25.3 1
EG51286 M 253 176 130 26.5 26.1 0
EG51287 M 247 178 134 26.3 26.4 3
EG51289 M 236 174 146 27.1 25.4 0
EG51298 M 215 178 139 25.5 23.3 0
EG51211 M 251 172 128 26.3 25.0 3
EG51212 M 234 176 132 26.9 23.3 1
EG51213 M 241 176 137 27.1 24.9 0
EG51214 M 229 180 138 26.2 24.3 1
EG51223 M 171 176 141 25.1 25.0 *
EG51228 M 236 177 150 26.6 25.7 0
208

Chapter 6:
Body mass regulation
ABSTRACT:
Numerous authors have noted that birds tend to gain mass in winter, most of
which is fat. These fat reserves are optimized in order to minimize both the risk of
starvation and other mass-dependent costs (e.g. predation risk, metabolism, risk of injury,
foraging and pathological costs). In addition, some authors have noted birds tend to gain
mass during the course of the day, with some species (those that feed on unpredictable
items) gaining mass rapidly during the early morning, while other species (those that feed
on a predictable food source) gaining mass evenly during the course of the day. I tested
the following three predictions (1): Rose-ringed Parakeets gained mass during the winter
in order to minimize their risk of starvation; (2) Rose-ringed Parakeets do not gain mass
rapidly during the course of the morning, as they regularly visit feeders (a predictable
food source); and (3) Parakeets engage in a bimodal feeding pattern in order to minimize
their predation risk. From 17 February 2001 to 17 February 2004, a total of 292 parakeets
were captured and data on mass and time of capture were recorded. Mass was related to
time of capture, wing length, day length, and month. Unexpectedly, Rose-ringed
Parakeets did not gain mass over the course of winter, although average mass was lowest
during late summer and autumn. The reason for this is unclear, although it may be that
Rose-ringed Parakeets do not associate the colder temperatures during winter with a
decrease in food supply. Rose-ringed Parakeets did not show a rapid morning increase in
209

mass, which is consistent with the hypothesis that species feeding upon predictable food
sources tend to put on mass throughout the day. Very few parakeets were caught during
mid-day, which is consistent with hypothesis that they are managing predation risk by
engaging in a bimodal feeding pattern (feeding primarily early in the morning and again
late in the afternoon).
210

INTRODUCTION:
Although there have been several studies on Rose-ringed Parakeets Psittacula
krameri, nearly all of these have focused on the species’ breeding behaviour (e.g. Lamba
1966, Simwat and Sidhu 1973, etc.) or on its potential for crop destruction (e.g. Reddy
1998, Mukherjee et al. 2000, etc.). Other aspects of its natural history (e.g. body mass
regulation) have been poorly studied, particularly in areas where it has been introduced.
Birds manipulate their body mass for a variety of reasons. Some species engage in
premigratory fattening (e.g. Heise and Moore 2003). Other species gain mass while
breeding (e.g. Franklin et al. 1999) or while moulting (e.g. Van der Winden 2002). Many
temperate species gain mass during winter (e.g. Lehikoinen 1987).
It is well known that many small passerines gain mass during the winter in order
to deal with the increased starvation risk associated with winter’s colder temperatures and
shorter days, particularly when they feed upon an unreliable food source (Blem and
Shelor 1986, Lehikoinen 1987, Haftorn 1989, Cresswell 1998, Gosler 2002, Rogers and
Heath-Coss 2003). Recently, authors have begun recognizing that this mass gain
represents a tradeoff of a bird’s risk of starvation balanced against costs such as mass-
dependent predation risk, mass-dependent metabolism, mass-dependent risk of injury,
mass-dependent foraging and pathological costs (Witter and Cuthill 1993).
Of all these costs, mass-dependent predation risk is the best studied. A number of
papers have modelled the best strategies for birds to employ in order to minimize both
their risk of starvation and their risk of predation (Hedenström 1992, Houston et al. 1993,
McNamara et al. 1994, Brodin 2001). Empirical evidence for these models has also been
found. For instance, it has been demonstrated that fat Blackcaps Sylvia atricapilla have a
211

shallower angle of ascent and a slower velocity than Blackcaps without a heavy fat load
(Kullberg et al. 1996) which suggests that they may be more at risk for predation. Gosler
et al. (1995) inferred that mass-dependent predation on Great Tits Parus major occurs, as
the average mass of Great Tits in Wytham Woods increased after Sparrowhawks
Accipiter nisus disappeared from Wytham Woods during the 1960s and then declined as
Sparrowhawks reappeared in the late 1970s and 1980s. A study on Blue Tit Parus
caeruleus fledging rates likewise found that the heaviest fledglings were more at risk
from Sparrowhawk predation than lighter fledglings (Adriaensen et al. 1998). Finally, it
has been demonstrated that Great Tits reduce their fat reserves in response to a perceived
increase in predation risk (Gentle and Gosler 2001).
Comparatively little work has been done on mass-dependent metabolism.
Theoretically, as a bird increases its mass, locomotion will incur increased energetic costs
(Witter and Cuthill 1993). In addition, the maintenance costs incurred by increased fat
accumulation may increase the basal metabolic rate (BMR), although subcutaneous
layers of fat may mitigate this by providing a layer of insulation and thus reducing BMR
(Witter and Cuthill 1993). Empirical evidence for this is lacking, however, although a
few studies have demonstrated that seasonal changes in BMR (which may or may not be
related to changes in fat storage) do occur (O'Connor 1996, Liknes et al. 2002).
More work has been performed on mass-dependent foraging costs. For instance, a
bird that forages at the tips of branches may theoretically suffer reduced foraging
efficiency and be forced to alter its foraging microhabitat selection as it gains mass
(Witter and Cuthill 1993). There is some evidence to support this theory, as birds that
have been fitted with radiotransmitters (which immediately increase their mass) have
212

shown altered feeding behaviours and survival rates (Bray and Corner 1972, Wanless et
al. 1989, Paton et al. 1991).
In theory, there may be further mass-dependent costs associated with increased fat
loads. For instance, mass-dependent risk of injury (where a bird’s risk of injury in a
collision or while landing increases with a concomitant increase in mass) is theoretically
possible (Witter and Cuthill 1993). Likewise, mass-dependent pathological costs may be
incurred, similar to the higher rates of mortality observed in obese people (Witter and
Cuthill 1993). However, these topics have not yet been thoroughly investigated in the
literature and empirical evidence is lacking.
In addition to the work done on body mass regulation over the course of a season,
further work has been done on how birds regulate their body mass over the course of a
day. Theoretically, birds should balance their risk of starvation against the risk of mass-
dependent predation by foraging early in the morning and late in the afternoon
(Bednekoff and Houston 1994, McNamara et al. 1994). A number of studies have
confirmed this bimodal feeding pattern (Haftorn 1989, Haftorn 1992, Cresswell 1998,
Lilliendahl 2002).
The relative importance of early morning feeding bouts versus afternoon feeding
bouts depends upon the predictability of a food resource and on the availability of
predator-free refuges. Species that feed on an unpredictable resource (e.g. Blackbirds
Turdus merula feeding on earthworms) rapidly gain mass in the morning in order to
minimize their chance of starvation over the course of a day (Cresswell 1998). In
addition, species that engage in hoarding (e.g. Willow Tit Parus montanus and European
Nuthatch Sitta europaea) also exhibit a rapid gain in mass during the morning hours. In
213

contrast, species feeding upon a more predictable resource (e.g. Lesser Spotted
Woodpeckers Dendrocopus minor feeding upon invertebrates in a dying tree) tend to feed
during late afternoon, presumably to reduce their risk of mass-dependent predation during
the day (Olsson et al. 2000).
In this chapter, I test the hypothesis that Rose-ringed Parakeets gain mass (and
energy reserves) during mid-winter when the risk of starvation increases due to shorter
days and colder temperatures. In addition, I also test the hypothesis that Rose-ringed
Parakeets put on their mass primarily in the afternoon as they frequently feed at bird
feeders (a reliable and predictable food source). Finally, I test the hypothesis that they
engage in a bimodal feeding pattern in order to minimize their risk of predation.
METHODS:
From 17 February 2001 to 17 February 2004, a total of 292 Rose-ringed Parakeets
were caught at 16 sites (12 in SW London, three in SE London, and one at the Isle of
Thanet; see Figures 1 and 2). Nearly all of these birds were caught at feeders, although a
single bird was caught at the roost in Esher. All birds were fitted with a metal ring and
adults were weighed on a Pesola spring balance (readable to 1 g). Time of capture was
recorded, as was mean temperature during the day and day length. Measurements were
taken on flattened wing length (which has been correlated with mass; see Gosler et al.
(1998)), bill length (in order to predict sex) number of yellow underwing coverts (in
order to predict sex) and mass. A stopped wing rule (readable to 1 mm) was used to
measure wing length.
214

A General Linear Model was constructed in order to evaluate the interactions of
various temporal (e.g. month, time of day, day length) as well as physical (e.g. wing
length, sex and presence of primary moult) variables that might affect mass. Backwards
deletion of variables was used in order to construct a minimum adequate model.
At a sample of locations, mist netting at feeders was carried out throughout the
day (from dawn till dusk) in order to evaluate whether parakeets exhibited a bimodal
feeding distribution at feeder. They were also grouped into three categories: captured
during the morning (from dawn till 10:00 AM), captured in the middle of the day (from
10:00 AM till 2:00 PM), and captured in the afternoon (from 2:00 PM till dusk). A χ 2 test
was then performed using these three categories to evaluate whether birds exhibited the
predicted bimodal feeding pattern (e.g. greater numbers of parakeets would be caught in
the morning and/or evening than during the middle of the afternoon).
All statistics were performed on SPSS 11.0 and results are presented as mean ±
standard error (except where noted).
RESULTS:
A minimum adequate model for the GLM was constructed using backwards
deletion of variables (see Table 1). This model demonstrates that mass is significantly
related to wing length, day length, month, and time of day (R 2 = 0.332, p < 0.001).
However, Rose-ringed Parakeets do not show the mid-winter fattening that is
characteristic of small passerines (see Figure 3). Mass is fairly constant throughout the
215

year with the exceptions of August, September and October when parakeets captured
coming to bird feeders were noticeably lighter (see Figure 4).
Known-sex birds (n = 95; 73 males and 22 females) did not differ significantly in
mass (t-test, t= 0.748, df = 93, p = 0.457). However, predicted sexes (based on the binary
logistic model derived in Chapter 5) did indeed differ significantly in mass (t-test, t =
2.709, df = 229, p = 0.007). Figure 5 provides a bar graph of mass by month for each sex.
The mass of mature males (those that have a rose-coloured neck ring) averaged 142.8 ±
1.3 g, while subadult males (those that lacked a rose-coloured neck ring) averaged 143.5
± 0.9 g. The mass of both was very similar from February through July (only 1 adult male
was caught from August through December; see Figure 6). The difference in mass
between the two was not significant (t-test, t = -0.400, df = 143, p = 0.690).
Relatively few birds were caught while moulting. The difference in mass between
moulting birds (140.2 ± 2.1 g, n = 22) and non-moulting birds (141.7 ± 0.6 g, n = 270)
was not significant (Mann-Whitney U = 2637.000, Z = -0.875, p = 0.381). Mass was not
significantly related to whether a bird was moulting and so this variable was deleted from
the GLM.
A total of 15 birds were recaptured (see Table 2). In general, the mass of these
recaptured birds did not differ greatly from their initial capture mass (e.g.13 of the 15
birds exhibited a mass change of < 5%). There were however, two exceptions. One bird
(ring EG51219) lost 11 grams between when it was ringed in July 2002 and when was
recaptured in February 2003. Another bird (ring EG50380) gained 11 grams between
when it was ringed in March 2002 and when it was recaptured in July 2002. These two
216

irds appear to fit the general trend shown in Figure 3; winter and spring mass tends to be
lower than July mass.
Although parakeets were caught throughout the day, relatively few parakeets were
caught during the middle of the day (see Figure 7). Dawn to dusk mist netting at feeders
demonstrated that this observed trend was significant (χ 2 = 31.167, d.f. = 2, p < 0.001;
Figure 8).
DISCUSSION:
The average mass recorded for the UK parakeets was somewhat greater than that
reported in the literature, but information on the mass of these birds in their native range
is scanty. The mass of five male specimens (P. k. borealis) from Gujarat, India, reported
in Ali and Ripley (1969) ranged from 104-139 g. Two males measured in Nepal weighed
136 and 143 grams (Diesselhorst in Roselaar 1985). A laying female in Nepal had a mass
of 158 g (Diesselhorst in Roselaar 1985).
The reason for this is unclear. It is possible that greater mass observed in
parakeets in the UK may be linked to the colder temperatures (relative to northern India).
Bergmann’s rule predicts that warm-blooded creatures living in colder climates tend to be
larger than warm-blooded creatures living in warmer climates (Begon et al. 1986). It is
also possible that parakeets that visit bird feeders may have a greater body mass, as has
been shown for other species (Rogers and Heath-Coss 2003).
Numerous authors have noted that small passerines tend to be heavier in winter
than during other seasons (Haftorn 1992, Witter et al. 1995, Lilliendahl 2002).
Theoretically, a bird should respond to the longer nights and decreased availability of
217

food sources during winter by increasing its fat stores (Gentle and Gosler 2001). It has
been demonstrated that a number of species gain mass over the course of winter,
presumably by altering the amount of fat stored (Blem and Shelor 1986, Lehikoinen
1987, Gosler et al. 1995, Hake 1996, Cresswell 1998). Psittacines are also capable of
manipulating their body mass and fat stores in response to environmental conditions. For
instance, the Kakapo Strigops habroptilus is able to adjust its mass by up to 100% (Clout
and Merton 1998).
Consequently, I expected to discover that Rose-ringed Parakeets gained mass
during the winter (as had been reported for small passerines) but instead found that mass
was nearly constant throughout the year with the exception of August, September, and
October when parakeets caught at feeders tended to have lower masses (see Figure 4).
The reasons for this are unclear. Perhaps mass-dependent predation is higher in the late
summer and early fall, as higher rates of mass-dependent predation have been linked with
smaller body masses (Gosler et al. 1995). However, although there are reports of Hobbies
Falco subbuteo and Sparrowhawks Accipiter nisus attempting to catch parakeets in the
UK, these attempts were unsuccessful (Pithon and Dytham 1999). The only successful
predation attempts recorded to date have been cats preying on parakeets at feeders (Witt,
pers. comm..) The lack of predators for this introduced species suggests the possibility
that parakeets are not regulating their body mass in response to mass-dependent
predation, although it is possible that they may be regulating their body mass in response
to perceived predation risk (e.g. there may be greater numbers of people and pets in
gardens during late summer and autumn).
218

As Rose-ringed Parakeets are neither small nor passerines, it is worth considering
how they compare with other non-passerines. (It should be noted though that parakeets’
mass overlaps with some of the passerines mentioned in the above studies. For example
Creswell’s (1998) study found that winter Blackbird mass ranged from 84-150 g, while
the mass of the parakeets in this study ranged from 110-180 g.). Rose-ringed Parakeets
are primarily herbivorous, and it is worth comparing them with other herbivorous species.
Greenland White-fronted Geese Anser albifrons gain mass in late winter prior to
engaging in spring migration (Fox et al. 2003). Likewise, Rock Pigeons Columba guinea
in the Western Cape of South Africa were heaviest during winter and lightest during
summer (Underhill and Underhill 1997).
In contrast, non-passerine carnivores did not necessarily show the same pattern of
winter fattening. The body mass of female American Kestrels Falco sparverius
(controlled for body size) in Pennsylvania remained essentially unchanged during the
period November to February, while the body mass of male kestrels (controlled for body
size) decreased during this time period. However, the body mass of Barn Owls Tyto alba
in northeastern France increases in early winter and then decreases (Massemin et al.
1997).
It is worth noting, however, that the studies on fat stores mentioned above have
focused on temperate species. Whether the conclusions that the authors have drawn about
fat storage are applicable to tropical and subtropical species as well remains to be seen.
Wintering Ovenbirds Seiurus aurocapillus in Jamaica, for instance, do not gain mass over
the course of the winter. Wintertime is the dry season in Jamaica, and a reduction in prey
biomass occurs as the season progresses (Strong and Sherry 2000). In theory, the longer
219

nights of winter as well as the ongoing reduction of food should prompt Ovenbirds to
increase their mass in order to reduce their risk of starvation. (It should be noted that
winter nights in Jamaica are 13 hours in length, compared with 11 hours during the
summer). Instead, they lose mass over the course of the winter (Strong and Sherry 2000).
A similar study on Swainson’s Warblers Limnothylpis swainsonii in Jamaica found that
they do not gain mass over the course of the winter, as their mass remains constant during
the winter months (Strong and Sherry 2003). Other studies on tropical birds have shown
that seasonal changes in fat reserves for many species are minimal at best; see Meijer et
al (1996) for an overview.
Whether Rose-ringed Parakeets are essentially a tropical species is an issue that
has not been resolved in the literature. Forshaw (1989) describes them as being abundant
in the Himalayas of Nepal up to an elevation of 365 metres. Roselaar (1985) describes
them as occurring in “tropical and subtropical low latitudes” occurring to an elevation of
1300 m in the Himalayas, and 1600m in the uplands of peninsular India. Juniper and Parr
(1998) state that Rose-ringed Parakeets are found to an elevation of 1600 m in Asia and
2000 m in Africa. In Pakistan, they are found in summer up to an elevation of 914 metres
(Roberts 1991). However, they have been observed during the winter in Quetta (Pakistan)
in below-freezing conditions (Roberts 1991).
However, prolonged exposure to sub-freezing conditions may be detrimental to
the health of these parakeets. Rose-ringed Parakeets introduced into New York City, NY
(USA) suffered from frostbite during the winter (Roscoe et al. 1976). Likewise, a
naturalized population of Rose-ringed Parakeets in Belgium has suffered mortality during
winter months (Temara and Arnhem 1996). This suggests that while these parakeets may
220

not be truly tropical, they seldom have experience with sub-freezing weather in their
native range.
Several authors have noted the link between temperature and midwinter fattening
strategies for birds (Blem and Shelor 1986, Gosler 2002, Kelly et al. 2002), although
whether cold temperatures are a proximate or ultimate cause of fattening have not been
clarified. Perhaps the relatively mild winter temperatures of tropical zones do not provide
the proximate cues for tropical species to increase their fat storage during this season.
However, when a tropical species is exposed to cold temperatures, it should increase its
fat reserves. A study on Zebra Finches Taeniopygia guttata suggests otherwise. Zebra
Finches kept outdoors in an aviary in Germany did not increase their mass over the
course of the winter. Instead, they increased their mass over summer, in preparation for
reproduction (Meijer et al. 1996). This suggests that cold temperatures may be an
ultimate (rather than a proximate) cause of increased fat reserves. Rose-ringed Parakeets
may not gain mass during the winter because, as a species that seldom experiences
below-freezing conditions in its native range, they do not recognize that cold weather
corresponds with decreased food availability.
The reason for this decreased mass during late summer and autumn is unclear.
Some authors have noted that moulting birds increase in mass (e.g. Heise and Moore
2003, Van der Winden 2002), particularly before engaging in migration. Since Rose-
ringed Parakeets are non-migratory, it was not unexpected that there was not a significant
difference between the mass of moulting birds and non-moulting birds.
The biological significance of a decrease in mass during late summer and autumn
of parakeets caught at feeders is uncertain. Perhaps the increased abundance of fruits and
221

seeds during this period may be responsible. Perhaps most Rose-ringed Parakeets switch
to feeding on fruits and seeds during this period, and only a few birds that are unable to
take advantage of this resource continue to come to feeders.
Some authors have also shown that birds increase their mass over the course of a
day (Haftorn 1992, Bednekoff and Houston 1994, Olsson et al. 2000). This prediction
was borne out for Rose-ringed Parakeets, as time of capture was a significant component
of the minimum adequate model GLM created to explain changes in mass.
As discussed earlier, birds that feed upon an unreliable food source should rapidly
gain mass over the course of the morning (Cresswell 1998). As parakeets feed upon a
stable food source (e.g. they frequent feeders), they should not exhibit a rapid mass gain
during the morning hours. This prediction was supported by data gathered during the
study – parakeets did not exhibit a rapid change in mass during the morning (see Figure
7). Their lack of a rapid morning increase in mass is consistent with the hypothesis that
they feed on a predictable food source.
Finally, I hypothesized that parakeets should exhibit a bimodal feeding pattern,
foraging early in the morning and late in the day in order to minimize their predation risk.
A significant bimodal pattern of feeding activity was observed over the course of study;
this result is consistent with other studies on the foraging patterns of small passerines
(e.g. Houston et al. 1993, Bednekoff and Houston 1994, McNamara et al. 1994,
Cresswell 1998, Olsson et al. 2000, etc). This result is also similar to what has been
reported in Rock Doves Columba livia another species that feeds primarily upon
vegetative matter (Henderson et al. 1992, Basco et al. 1996).
222

In conclusion, mass in Rose-ringed Parakeets is related both to wing length and
the month. Other physical factors examined (such as bill length, tail length, toe length,
etc.) were not significant components of the minimum adequate GLM needed to explain
mass. Although other authors have shown that some birds exhibited a significant change
in mass over the course of the day, no such change was apparent in Rose-ringed
Parakeets. Finally, Rose-ringed Parakeets show the same bimodal feeding pattern (i.e. a
peak in feeding activity during the early morning and late afternoon) that has been found
in other bird species.
223

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230

TABLES:
Table 1: A minimum adequate model for mass (R 2 = 0.332). This GLM was constructed
through backwards deletion of variables 1 .
Source Type III Sum of
Squares
1 Variables initially included in the model also included predicted sex and presence of
primary moult as well as interactions.
ß df Mean Square F Sig.
Corrected Model 6898.671(a) 25 275.947 3.763 0.000
Intercept 671.416 1 671.416 9.157 0.003
Time of capture 301.532 0.009 1 301.532 4.112 0.044
Wing length 1654.425 0.548 1 1654.425 22.564 0.000
Day length 368.495 -0.063 1 368.495 5.026 0.026
Month 2315.611 Jan = -3.169
Feb = 23.012
Mar = 12.177
Apr = 18.432
May = 34.787
Jun = 30.719
Jul = 47.295
Aug = 30.662
Sep = -0.521
Oct = 14.484
Nov = -4.969
Dec = 0
11 210.510 2.871 0.002
Month * time 1797.869 Jan * time = -0.001
Feb * time = -0.020
Mar * time = -0.006
Apr * time = -0.005
May * time = -0.012
Jun * time = -0.004
Jul * time = -0.014
Aug * time = -0.015
Sep * time = 0.006
Oct * time = -0.015
Nov * time = 0.010
Dec * time = 0.000
11 163.443 2.229 0.015
Error 13857.992 189 73.323
Total 4353461.040 215
Corrected Total 20756.663 214
231

Table 2: A summary of recaptures and their mass. In general, the mass of recaptured
birds was very similar to the mass when initially captured. Only two birds (EG51219 &
EG50380) exhibited a large mass change (i.e. > 5%).
Ring Date Mass Date Mass mass change (in g)
EG50304 3/3/2001 131 19/5/2001 129 -2
EG50314 31/3/2001 140 19/5/2001 137 -3
EG50353 17/7/2001 137 28/9/2001 136 -1
EG50343 19/5/2001 136 18/10/2001 135 -1
EG50302 17/2/2001 145 12/11/2001 148 3
EG50348 9/6/2001 153 20/5/2002 151 -2
EN37824 13/8/2001 135 20/5/2002 130 -5
EG50341 19/5/2001 141 20/5/2002 146 5
EG51219 21/7/2002 166 21/2/2003 155 -11
EG50328 29/4/2001 153 24/6/2002 148 -5
EG50380 12/3/2002 124 22/7/2002 135 11
EN37823 13/8/2001 127 22/7/2002 131 4
EG51244 3/4/2003 142 8/5/2003 147 5
EG51246 3/4/2003 140 8/5/2003 144 4
232

FIGURES:
Figure 1: Locations in the Greater London area where Rose-ringed Parakeets were
ringed. The city of London is provided as a reference point. A total of 287 parakeets were
caught at 15 locations.
233

Figure 2: Locations at the Isle of Thanet, Kent, where Rose-ringed Parakeets were
caught. The city of Margate is provided as a reference point. Five parakeets were caught
at one locations.
234

Figure 3: Mass changes throughout the year (n = 292). No trend in mass is immediately
apparent. Note also that most birds were caught from February to July. This reflects
changes in capture effort (i.e. a greater effort was made to capture birds from February to
July) rather than changes in the number of parakeets coming to feeders.
Mass (g)
190
180
170
160
150
140
130
120
110
100
0 1 2 3 4 5 6
Month
7 8 9 10 11 12
235

Figure 4: Average mass for each month (n = 292). Note that parakeets are lightest in
August (135.6 ± 2.8 g, n = 9), September (136.2 ± 4.1 g, n = 11), and October (136.8 ±
2.9 g, n = 4).
Mass (g)
155
150
145
140
135
130
125
120
January
February
March
April
May
June
July
August
September
October
November
December
Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
N = 15 24 36 47 58 20 37 9 11 4 5 31
236

Figure 5: A bar graph of mass by month for each predicted sex (n = 144 males and 87
females; insufficient information was gathered to predict the sex of the other birds).
Mass (g)
165
160
155
150
145
140
135
130
125
120
# of
Males
# of
Females
January
February
March
April
May
June
July
August
September
October
November
December
Month
Male Mass
Female Mass
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
3 13 21 29 31 13 13 3 3 3 1 12
3 8 13 11 14 2 18 1 6 1 3 9
237

Figure 6: A bar graph of mass by month for mature males (those with a pink-coloured
neck ring) and immature males (those that lack a pink-coloured neck ring).
Mass (g)
# of
adult
males
165
160
155
150
145
140
135
130
125
120
# of
subadult
males
January
February
March
April
May
June
July
August
September
October
November
December
Month
Mature Male
Immature Male
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1 2 11 13 17 7 2 1 0 0 0 0
2 11 10 17 14 6 11 3 2 3 1 12
238

Figure 7: A scatterplot of mass and time of day. Rose-ringed Parakeets tended to have a
greater mass during the afternoon (142.8 ± 0.9 g, n = 128) than during the morning (140.7
± 1.0 g, n = 87), and time of capture was a significant component of the minimum
adequate GLM used to predict mass. No birds were caught twice during the same day.
Mass (g)
190
180
170
160
150
140
130
120
110
100
400 600 800 1000 1200 1400 1600 1800 2000 2200
Time
239

Figure 8: A histogram of capture times (n = 158) at locations where mist netting was
carried out throughout the day. Note the bimodal pattern; relatively few birds were caught
during the middle of the day, from 1000 to 1400.
240

Chapter 7:
Population & spatial modelling
ABSTRACT:
It is well known that introduced species have the potential to cause both economic and
biological damage in countries where they have been introduced. Rose-ringed Parakeets
have recently been demonstrated to cause economic damage to crops in the UK, reducing
a traditional vineyard’s output from 3,000 bottles of rosé down to 500 bottles in 2002. In
order to evaluate how rapidly Rose-ringed Parakeets might expand across the country,
four asymptotic wave front population expansion models were examined, and compared
against observed expansion rates. In addition, population models were constructed in
order to evaluate how rapidly the population would increase in the Greater London area
and the Isle of Thanet, and six different management options were evaluated with these
models. Rose-ringed Parakeets were found to be spreading at a rate of approximately 0.4
km/yr using available data from the Greater London area. However, they were not found
to be expanding their range on the Isle of Thanet, perhaps because of the coarse grid size
used. Three of the four wave front models overestimated the expansion rate, but one
model incorporating life history elements projected range expansion to occur at the rate
of 0.4 km/yr that fit the observed data. Population models for the Greater London area
revealed that the population there is increasing at a rate of 30% per year, while
populations on the Isle of Thanet are increasing at a rate of 15% per year. In order to
241

prevent these populations from increasing further, it is necessary to harvest 30% of the
population annually. As the parakeets gather into large communal roosts throughout the
year, such harvesting would be most effective at these communal roosts.
242

INTRODUCTION:
Introduced species may have a negative impact on biodiversity (With 2002) and
may cause economic damage as well (Owen 1990, Fritts 2002). As discussed in previous
chapters, Rose-ringed Parakeets have the potential to have a negative impact on both
native species and crops. Rose-ringed Parakeets begin breeding in early March, which is
considerably earlier than native species and could conceivably outcompete native species
for nest cavities. In addition, they are considered a serious crop pest on the Indian
subcontinent and could become a crop pest in the UK as well (see Chapter 1).
To date, there is no evidence that Rose-ringed Parakeets are indeed having an
effect on native cavity-nesting birds (see Chapter 4). However, there is evidence that they
are beginning to adversely affect crops. An article carried in the Kingston Guardian in
October 2002 describes the damage that parakeets caused to a vineyard in Painshill Park
(Surrey) (Saines 2002). Traditionally, this vineyard produces 3000 bottles of rosé wine
for consumption. However, in 2002 the parakeets decimated the grape crop, and only 500
bottles could be produced (Saines 2002).
Successful invasions of introduced species typically proceed in the following
manner. First, they are introduced (either intentionally or accidentally) into a new area
(Andow et al. 1990, With 2002). Secondly, this introduced population begins reproducing
and becomes self-sustaining (Andow et al. 1990, With 2002). Finally, the population
begins spreading (Andow et al. 1990, Hastings 1996a, Hastings 1996b, With 2002).
Rose-ringed Parakeets are now in the expansion phase. They have increased at a
rate of 25% per year since the mid-1990s (see Chapter 2). In addition, they have been
gradually expanding their range (Butler 2002). As they are now proving to be a crop pest
243

in the UK, it is imperative to assess their eventual population and distribution in the UK,
as well as potential management options that could be used to regulate the population.
Since it is believed that attempts to control an introduced population will be more
successful, at the earliest stages of an introduction (Williamson and Brown 1986), any
efforts to control Rose-ringed Parakeet numbers should be undertaken as soon as
possible.
In order to evaluate the rate of spread and population growth of Rose-ringed
Parakeets in the UK, it is necessary to engage in both population and spatial modelling.
Population modelling
A population viability analysis (PVA) is a mathematical model to predict either
the risk of extinction or the population size during a specified period of time (Cross and
Beissinger 2001). Typically, it is applied to populations of threatened or endangered
species to evaluate the effects of potential management techniques (e.g., Haig et al. 1993,
Bustamante 1996, Brook and Kikkawa 1998, Reed et al. 2002).
Ideally, PVAs should be conducted on well-understood populations with long-
term data on birth rates, mortality rates, population structure, and catastrophes (Brook et
al. 1999). However, in practice, much of this information is not available and so
researchers have to utilize data that are often little more than a “best guess” (Drechsler et
al. 1998, Burgman 2000). Nonetheless, PVAs are more useful in evaluating management
decisions than other techniques (Burgman 2000).
In order to evaluate which life history attribute managers should focus their
attention on, sensitivity or elasticity analysis is typically performed (Drechsler 1998, de
244

Kroon et al. 2000). A sensitivity analysis quantifies the absolute change in population
growth rate (λ) based on a change in a parameter (Drechsler 1998). In contrast,
elasticities quantify the proportional change in population growth rate (λ) based on a
change in a parameter (Cross and Beissinger 2001).
the form:
The simplest PVAs are based on Leslie matrices. These matrices are generally of
Bn =
⎡ 0
⎢
⎢
S0
⎢ 0
⎢
⎣ 0
F
0
S
1
1
0
F
2
0
0
S
2
F3
⎤
0
⎥
⎥
0 ⎥
⎥
0 ⎦
Where Fx is age-specific fecundity, Sx is age-specific survival, and Bn is the population
size. This Leslie matrix can be projected forward in time in order to estimate the
population size over a specified time period.
In theory, a PVA could be conducted on an invasive species as well, although no
such studies have apparently been published (although there are papers on reintroduction
efforts – see Lubina and Levin 1988, Bustamante 1996, South et al. 2000). Such a study
would be very useful in examining the potential growth of the species, as well as the
response of the species to different management regimes.
Spatial modelling
Spatial models are mathematical models that simulate population expansion. The
simplest of these models are diffusion models. Fisher’s (1937) work on the spread of
genes was expanded by Skellam (1951) to model the spread of organisms. Diffusion
(1)
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models have been used to study the invasions of muskrats (Ondatra zibethicus) in
Europe, small cabbage white butterfly (Pieris rapae) in North America, and the cereal
leaf beetle (Oulema melanopus) in North America (Andow et al. 1990). These diffusion
models are generally of the form:
2
δn
δ n
= rn + k
(2)
2
δt
δx
Where n is population size, t is time, r the intrinsic rate of increase, x is radial distance, k
the diffusion constant (Williamson and Brown 1986).
In the absence of Allee effects, these diffusion models predict an asymptotically
constant rate of spread:
C = 2rs
(3)
Where C is the rate of spread and r is the rate of increase, and s is the diffusion constant
(van den Bosch et al. 1992).
However, diffusion models operate under a number of assumptions that may not
necessarily be true in a biological system. For instance, it is assumed that all individuals
are identical in mortality and reproduction (Hengeveld and van den Bosch 1990, Lensink
1998). In addition, it is assumed that individuals move at random throughout their range
(Hengeveld and van den Bosch 1990, Lensink 1998). Finally, it is assumed that density-
dependence is logistic (Hengeveld and van den Bosch 1990). In addition to the problems
246

with the biological relevance of these assumptions, estimating the diffusion parameter
can be difficult (van den Bosch et al. 1992).
For these reasons, many authors now use spatial models that incorporate life
history parametres. For instance, the following equation was used by Hengeveld and van
den Bosch (1990) to model the expansion of the Eurasian Collared Dove Streptopelia
decaocto in Europe:
σ
C ≈ ( 2ln
Ro
)
(4)
µ
Where C = expansion rate, σ 2 = is the marginal dispersal density variance, µ = the mean
age at reproduction, and R0 = lifetime reproductive rate (Hengeveld and van den Bosch
1990). However, this is only suitable for R0 ≤ 1.5 (van den Bosch et al. 1992). For species
with R0 ≥ 1.5 (e.g. House Sparrow Passer domesticus), van den Bosch et al. (1992)
suggests that the following equation be used:
σ ⎧ υ 2 1 ⎫
C ≈ ( 2ln
Ro
) ⎨1
+ [( ) − β+
γ ln Ro
⎬
µ ⎩ µ 12 ⎭
Where υ 2 = the variance of the age at child bearing, γ is the kurtosis (the degree to which
the distribution is peaked) of the marginal dispersal density, and β is a measure of the
interaction between dispersal and reproduction.
Finally, there are integrodifference equations, which are based on a discrete-time
dispersal model (Neubert and Caswell 2000). Dispersal kernels give the probability of an
(5)
247

individual occurring in a location based on its initial location (Neubert and Caswell
2000). The following integrodifference equation (based on a structured population) has
been proposed by Neubert and Caswell (2000)
∞
n(x, t + 1) = ∫
−∞
[K(x-y)o Bn]n(y,t) dy (6)
Where Bn(y) is the density-dependent population matrix at location y and K is the
dispersal kernel matrix.
Neubert and Caswell (2000) argue that this integrodifference equation more
accurately reflects the underlying processes of a biological invasion than diffusion
models. In addition, because this model is based on matrices, it is possible to calculate
elasticities and perform a sensitivity analysis in order to evaluate which life history
parametres are most important in a species’ expansion (Neubert and Caswell 2000).
In addition to theoretical models presented above, it is also possible to calculate
expansion rates empirically by comparing maps of a species range during different
decades. For instance, during 1988-91, the British Trust for Ornithology (BTO) carried
out a systematic survey of all breeding birds in the UK (Gibbons et al. 1993). Data were
gathered for a series of 10 km x 10 km (100 km 2 ) squares on the presence of bird species
in that square, as well as whether evidence of breeding was found. If the data gathered
over the course of this study were plotted in the same manner, it would be possible to
estimate how rapidly Rose-ringed Parakeets were expanding their range in the UK.
248

Chapter objectives
For this chapter, range expansion will be calculated using diffusion models that
incorporate life history parametres as well as the integrodifference equations. They will
then be compared to empirical data on range expansion in order to test the hypothesis that
these mathematical models accurately predict range expansion rates.
In addition, a PVA model will be constructed in order to examine the rate of
population growth. Parametres in this model will then be examined in order to identify
which parameter(s) might best be modified in order to manage the population.
METHODS:
Population modelling
Multiple Leslie matrices were constructed in order to model Rose-ringed Parakeet
populations. Reproductive rates for this species in the UK have been established (see
Chapter 5), but age-specific survival rates (or indeed, any survival rates) for this species
are not available in the literature. In general other parrot species show higher mortality as
juveniles than as adults (e.g. Snyder et al. 1987, Spreyer and Bucher 1998, Brightsmith
1999). Mortality rates could not be calculated based on data gathered during this study,
due to the limited number of recaptures (see Chapter 6). Consequently, mortality rates for
two well-studied parrot species (Puerto Rican Parrots Amazona vittata and Monk
Parakeets Myiopsitta monachus) were incorporated into Leslie matrices. (Although these
249

two species differ considerably in ecology and body size from Rose-ringed Parakeets,
age-specific mortality rates for species more similar in ecology and body size to Rose-
ringed Parakeet have not been published.) Age-specific fecundities were then calculated
based on these survival rates. See Table 1 for a summary of each model.
Separate models were constructed for the Greater London population and the Isle
of Thanet population. Parakeets in the Greater London area have been observed to breed
while still age 2, whereas Rose-ringed Parakeets at the Isle of Thanet do not begin
breeding until age 3 (see Chapter 5). In addition, the Isle of Thanet population has not
grown as rapidly as the Greater London population (see Figure 1), so a model that fits the
observed population growth in the Greater London area would not be suitable to model
the Isle of Thanet population and vice versa.
Count data are available for each of these populations for the period 1996-2001.
The models were set at the 1997 population size and a stable age distribution was then
calculated. These populations were then projected to 2001. The mortality rates that gave a
model population closest to the actual population size during 2001 were adopted as the
mortality rates for Rose-ringed Parakeets in each area. Once the models were constructed,
they were subjected to sensitivity and elasticity analysis in order to identify the parameter
that has the greatest effect on their population growth.
The following formula was used for the sensitivity analysis
dλ viwj
=
dai
, j w,
v
(7)
250

Where the eigenvector w is the stable age distribution, the eigenvector v is the relative
reproductive value, and the dominant eigenvalue λ is the long-term growth rate
The following formula was used to calculate elasticities
% change in λ aijviwj
= (8)
% change in ai
, j λ v,
w
Once these simple models had been constructed, evaluation of management
regimes was initiated. Six management regimes were evaluated: (1) harvesting 10% of
the adult population each year; (2) harvesting 10% of the entire population each year; (3)
harvesting 30% of the adult population each year; (4) harvesting 30% of the entire
population each year; (5) harvesting 50% of the adult population each year; and (6)
harvesting 50% of the population each year.
Spatial modelling
Data from the 1988-91 Breeding Bird Atlas were obtained from the BTO. These
data show the 10 km x 10 km (100 km 2 ) squares where Rose-ringed Parakeets were
reported during 1988-91. In addition, evidence of breeding was reported from some of
these squares.
From the spring of 2001 to 2003, searches for nests of this species were carried
out in the southern UK. In addition, requests for sightings were made through various
media (newspaper, radio, and internet). Locations where this species was seen routinely,
251

as well as confirmed breeding locations were mapped onto the same grid of 10 km x 10
km squares that the BTO utilized.
Because Rose-ringed Parakeets are frequently kept as pets, escapees may occur
anywhere. Indeed, the 1988-91 Breeding Bird Atlas shows numerous sightings
throughout the UK. However, these escapees seldom survive and reproduce.
Consequently, in order to evaluate changes in the area occupied two different criteria
were used. The first technique was to use breeding records that were in adjacent squares,
as these parakeets should theoretically form a single, breeding population. The second
technique was to use sightings from adjacent squares where Rose-ringed Parakeets were
seen under the assumptions that breeding birds may be few and far between at the edge of
their range.
These estimates of range expansion were then tested against four models of range
expansion. Estimates of range expansion were calculated form equations 3 and 4. In
addition, a model was constructed based on Neubert and Caswell’s (2000) model.
However, their model was a stage-structured model, rather than an age-structured model.
The demographic part of a model involving Rose-ringed Parakeets is simply a Leslie
matrix model.
The dispersal kernel matrix is:
B(n) =
⎡ 0
⎢
⎢
S0
⎢⎣
0
0
0
S
1
F1
⎤
0
⎥
⎥
S ⎥ 2 ⎦
(9)
252

K(n) =
⎡ δ ( x)
⎢ 1 −|
x|
⎢ e
⎢2α
⎣ δ ( x)
α
δ ( x)
δ ( x)
δ ( x)
δ ( x)
⎤
⎥
δ ( x)
⎥
⎥
δ ( x)
⎦
1 | x|
The dispersal kernel is assumed to be a Laplace distribution ( e
2α
−
α
). The delta
function (δ(x-y)) is roughly zero if x ≠ y, infinite if x = y, and integrates to 1. A
probability of 1 indicates that an individual remains in the same location. Alpha (α) is the
mean dispersal distance. It was estimated from breeding records from county bird reports
using the same methods as Lensink (1998).
In order to calculate a matrix of the moment-generating functions of the dispersal
kernels (M(s)), it is necessary to integrate K(s) according to the following formula (see
Neubert and Caswell (2000) for a full derivation):
Therefore,
M(s) = ∫ ∞
M(s) =
−∞
⎡ 1
⎢ 1
⎢
⎢
2α
⎣ 1
(10)
K(ζ)esζ dζ (11)
The matrix H(s) is then generated by B º M, where the symbol “º” stands for the
2 2
s
Hadamard product. The resulting matrix has the following form:
1
1
1
1⎤
⎥
1⎥
1
⎥
⎦
(12)
253

H(s) =
⎡ 0
⎢ S0
⎢ 2
⎢1−
α s
⎢⎣
0
2
0
0
S
1
F1
⎤
⎥
0 ⎥
⎥
S 2 ⎥⎦
The asymptotic wave speed of the expansion rate can be found according to the following
formula:
(13)
1
c() s = ln ρ1(
s)
(14)
s
In order to find the minimum wave speed (c*), it is necessary to find the value of s that
makes the following equation true:
Where ρ1 is the largest eigenvalue of H(s).
⎡1
⎤
c* = min 0

In addition, they provide a method for calculating the elasticity of the minimum
asymptotic wavefront (c*) to changes in demographic parametres:
RESULTS:
Population modelling
aij dc * 1 ⎡hij
dρ
⎤
1
= ⎢ ⎥
c * daij
ln ρ1
⎢⎣
ρ1
dhij
⎥⎦
Incorporating Monk Parakeet age-specific mortality rates into the Leslie matrix
consistently yielded a population estimate that was too low for the Greater London
population (see Figure 2). Incorporating the age-specific mortality rates for Puerto Rican
parrots yielded a much closer estimate of the true population size (see Figure 2).
However, the age of first reproduction in the Puerto Rican parrot is at age 4 (Snyder et al.
1987); the age of first reproduction of in Rose-ringed Parakeets is generally at age 3
(Lamba 1966) but can be at age 2 (pers. obs). The following Leslie matrix (Model 1)
obtained the best match to the observed population (see Figure 2):
Model 1 =
⎡ 0
⎢
⎢
0.
675
⎢⎣
0
0
0.
775
1.
283⎤
0
⎥
⎥
0.
875⎥⎦
In contrast, Rose-ringed Parakeets at the Isle of Thanet have not been observed
breeding until fully mature (age 3). A comparison of the models shows that the age-
specific mortality associated with Puerto Rican Parrots results in a population projection
that is too large (see Figure 3). Similarly, Model 1 results in a population projection that
0
(17)
(18)
255

is too large (see Figure 3). In contrast, the model using Monk Parakeets comes much
closer although it is too low. The following Leslie matrix results in the best fit for the
observed population.
Model 2 =
⎡ 0
⎢
0.
65
⎢
⎢ 0
⎢
⎣ 0
0
0.
75
0
0
0
0.
85
1.
235⎤
0
⎥
0
0.
85
Both models were set to the population size in 2001 and projected 50 years into
the future. No attempts to model density-dependent effects were included, as this
projection merely serves to demonstrate the exponential growth that this species is
capable of (see Figures 4 and 5). A summary of relevant life-history parametres is
presented in Table 2.
0
Sensitivity analyses were performed on both matrices. The sensitivity analysis for
both the Greater London population and the Isle of Thanet demonstrated that adult
survival had the largest effect on population growth, rather than subadult survival or
reproductive rates (see Figure 6). Since the largest eigenvalues are consistently adult
survival rate, any efforts to manipulate the numbers of this species should focus on
this parameter.
Six management regimes were evaluated. In order to prevent the Rose-ringed
Parakeet population in Greater London from increasing, 30% of the entire population
must be harvested each year (see Figure 7). Similarly, a harvest of 30% of the population
0
⎥
⎥
⎥
⎦
(19)
256

is required in order to prevent the population at the Isle of Thanet from increasing (see
Figure 8).
Spatial modelling
During 1988-91, multiple populations of Rose-ringed Parakeets were found to be
breeding in the UK. Parakeets were found to be breeding at the Isle of Thanet, Brighton,
Greater London, and there were isolated breeding records for West Sussex (see Figure 9).
During 2001-03, parakeets were found breeding in the Greater London area, the Isle of
Thanet, and Studland. Breeding parakeets were not reported from Brighton, although
occasional sight records are still recorded (see Figure 10).
The population at the Isle of Thanet has increased considerably during the 1990s
and early 21 st century. However, the population has not expanded its range beyond the
200 km 2 that it occupied during the 1988-91 Breeding Bird Atlas. The population in the
Greater London area has similarly increased since the 1988-91 Breeding Bird Atlas but
its range has expanded. During the 1988-91 Breeding Bird Atlas, breeding parakeets
occupied an area of 800 km 2 . During 2001-03, their breeding range was found to occupy
an area of 1200 km 2 . From 1991 to 2003, Rose-ringed Parakeets in the Greater London
area expanded their breeding range by approximately 3.6 km in all directions over 12
years or approximately 299 m in all directions per year.
During the 1988-91 Breeding Bird Atlas, parakeets were seen in a total of 15
adjacent squares, in the Greater London area, occupying an area of 1500 km 2 . During
2001-03 they were seen in a total of 23 squares in the Greater London area, occupying a
257

ange of 2300 km 2 . During the period from 1991 to 2003, Rose-ringed Parakeets
expanded their range by approximately 5.2 km in all directions over 12 years or
approximately 434 m per year.
The simple diffusion models indicate that the square root of the area occupied
should increase at a linear rate over time (Skellam 1951). This prediction has been born
out for several species undergoing range expansions (Skellam 1951, Lubina and Levin
1988). Similarly, this appears to be true for Rose-ringed Parakeets as well, although the
effect is only weakly significant (see Figure 11).
In order to estimate σ 2 (variation in dispersal distances) for equations 2 and 3, it is
first necessary to estimate α (mean dispersal distance). A histogram of dispersal distances
can be seen in Figure 12. On average, new breeding locations reported to the county bird
reporters were approximately 4 km from existing breeding locations.
Of the four models tested, it was found that Equation 3 provided the most realistic
approximation of the rate of spread in the Greater London area (0.443 km/yr, compared
to an observed 0.434 km/yr; see Table 3). Similarly, the same equation provided the
closest approximation to the observed rate of spread (or lack thereof) at the Isle of
Thanet. The integrodifference equations (equation 5) produced the most optimistic
projection of expansion rates (2.3 km/yr; see Table 3).
However, the integrodifference equations utilized matrices, which can thus be
evaluated for the effects of demographic parametres on dispersal rates. Using equations
16 and 17, it was found that adult survival rates had the greatest effects on dispersal.
Therefore, efforts to reduce dispersal rates should focus on reducing adult survival (see
Figure 13).
258

DISCUSSION:
Of the four wavefront models tested, the model by van den Bosch et al. (1992) for
species with R0 ≤ 1.5 (equation 3) most closely resembled the actual range expansion
observed. However, it should be noted that there appears to be an error in the derivation
of this formula. The authors state that a simple diffusion model predicts that the
asymptotic wavefront of invasion should be equal to equation 2. They then claim that
And
In order to derive equation 3.
ln R0
r = (20)
µ
σ 2
s =
(21)
µ
However, the identity of r was not represented correctly in equation 20. The
identity of r is actually:
Where T is the generation time (Gotelli 2001).
ln R0
r = (22)
T
Equation 5 (the integrodifference equation) was the least accurate at predicting
the asymptotic wave speed of an invasion front. In part, this may be due to difficulties in
estimating α, the mean dispersal distance. Measuring the distance from new breeding
locations mentioned in the literature to previously-established breeding locations may
259

tend to overestimate the mean dispersal distance, as it does not capture the individuals
that remained to breed in the same area where they were born. Mean dispersal distances
measured in other Psittaciformes are very short (e.g. 95% of Green-rumped Parrotlets
Forpus passerinus dispersed < 500 m from their natal site; Sandercock et al. 2000).
Using this equation with α = 1.230 (the average dispersal distance of a similar species,
the Monk Parakeet Myiopsitta monachus; see Spreyer and Bucher 1998) yields a much
closer estimate of range expansion rate (see Table 3).
The lack of range expansion in the Isle of Thanet population may be an artifact of
the sampling protocol used. Records were mapped onto a relatively coarse grid (10 km x
10 km). Consequently, very small annual range expansions (as predicted by the various
models tested) may not be detectable.
The rapid population growth exhibited by Rose-ringed Parakeets during the last
decade has not been marked by a rapid expansion in range. In some species (e.g. Eurasian
Collared-Dove Streptopelia decaocto) range expansion can be quite fast (i.e. 43.70 km
per year; Hengeveld and van den Bosch 1990). Other species may expand at a slower
rate. Egyptian Geese (Alopochen aegyptiacus) in the Netherlands expanded at rates of
about 3.0 km/yr (Lensink 1998). Many species initially exhibit a slower period of range
expansion before switching to a faster rate of range expansion (Hastings 1996a). Cattle
Egrets Bubulus ibis exhibited this pattern (van den Bosch et al. 1992), as did Egyptian
Geese Alopochen aegyptiacus (Lensink 1998), and California sea otters Enhydra lutris
(Lubina and Levin 1988).
The reason for this pattern remains unresolved. In general, integrodifference
equations predict that it is long-distance dispersal that drives asymptotic expansion wave
260

fronts (Neubert and Caswell 2000, With 2002), so an increase in long-distance dispersal
rates is required in order for expansion rates to increase. Some authors have suggested
that there is selection for individuals with a higher propensity for dispersal (Travis and
Dytham 2002). Other authors have explained this pattern by means of transient Allee
effects (Lewis and Karieva 1993).
Range expansions are seldom uniform in all directions (Gammon and Maurer
2002). Although most models assume homogenous habitats, habitats tend to be
heterogeneous (Gammon and Maurer 2002, With 2002). It has been suggested that
individuals dispersing from a population seek out preferential habitats and settle there,
leading to a non-random dispersal pattern (Gammon and Maurer 2002). Consequently, it
is not unexpected that non-random dispersal is encountered in some species. With regards
to Rose-ringed Parakeets, a comparison of the data obtained from 2000-2003 with the
1988-1991 data shows that parakeets in the Greater London area expanded primarily to
the north and the west with very little expansion evident in the south and the east.
Some species are capable of long-range propagule dispersion. For example,
introduced Starlings Sturnus vulgaris and House Sparrows Passer domesticus in the US
established small populations well in advance of their main invasion wavefront (Gammon
and Maurer 2002). As mentioned previously, integrodifference equations reveal that it is
these rare long-distance dispersal events that drive expansion rates. Although Rose-
ringed Parakeets do not appear to be prone to such long-distance wandering, they are
frequently kept as pets, and it is possible that future releases and/or escapes may serve to
increase the asymptotic expansion wave front rate.
261

The models tested in this chapter did not include density-dependent effects.
Efforts have been made to include density dependence in diffusion models of asymptotic
wave fronts of invasions. Surprisingly, models with density-dependence yield the same
estimates for the asymptotic wave front speed as models without density-dependence
(Hastings 1996b). This may be due to the fact that population density is typically low
near the edge of an expanding range and so density-dependence does not play a large role
(Hastings 1996b).
Population regulation
The population models of Rose-ringed Parakeets demonstrate the potential for
rapid increase in the population of this species. While the population is increasing
exponentially, the range is expanding relatively slowly (approximately 0.4 km/yr). This
situation will lead to a high density of parakeets in areas where they occur. The prospect
of high parakeet densities may be not be a welcome one to farmers in Surrey, Middlesex,
Berkshire, Buckinghamshire and Kent as Rose-ringed Parakeets have already
demonstrated that they can have a negative impact on crops in Britain. Consequently, an
evaluation of potential management techniques is in order.
The debate over whether to manage Rose-ringed Parakeet populations in the UK
dates back to shortly after the first breeding individuals were reported. Some people
expressed strong opinions that Rose-ringed Parakeets should be eradicated as soon as
possible as they could potentially become a crop pest, compete for nest cavities with
native species, or transmit diseases to humans (e.g. England 1974, Tozer 1974). In
contrast, others argued that they greatly enjoyed seeing free-flying parakeets in Britain
262

(e.g. Campbell 1975). This debate continues today, with many people desiring the
complete eradication of Rose-ringed Parakeets in the UK, while other people gain a great
deal of satisfaction from seeing them in parks or at garden bird feeders (pers. obs.).
First of all, however, it should be noted that any effort to manage Rose-ringed
Parakeet numbers faces some serious obstacles. To start with, the largest numbers of
Rose-ringed Parakeets are typically found in suburban gardens and parks. Any efforts to
manage the population in these areas would need to take into account the relatively high
public visibility of such an effort. In addition, it is likely that people who enjoy seeing
parakeets in their local parks and at their garden feeders would object to any plan to
reduce their numbers.
In addition, Rose-ringed Parakeets are protected under the Wildlife and
Countryside Act 1981 (Holmes and Simmons 1996). This act prohibits the taking of wild
birds and their eggs. The Act is worded very broadly, with a wild bird being defined as
“any bird of a kind which is ordinarily resident in or is a visitor to Great Britain, in a wild
state, but does not include poultry or, except in sections 5 and 16, any game bird”
(Holmes and Simmons 1996). However, the law allows “authorized persons to kill, injure
or take certain species of bird causing serious agricultural damage under the terms of a
general license issued by the Government” (Holmes and Simmons 1996). However, it
should be noted that such licenses permit shooting in aid of scaring and not as a means of
population control (Feare pers. comm.).
A variety of potential techniques have been suggested in order to control feral
parrot populations. The state of California, for instance suggests the following possible
methods: chemical agents (including anesthetics and toxicants), entanglements (e.g. bird
263

lime), shooting, tape playbacks (to lure parakeets in), traps, waternets (e.g. spraying birds
with a high-pressure stream of water, night-lighting, electrocution (judged to be too
hazardous in California) and a bounty system (State of California 1976).
In addition to these techniques, it may also be possible to harvest chicks. At the
Isle of Thanet, there are several trees with a “V” notched into them, where parakeet
chicks have apparently been harvested (Verrell pers. comm.). As Rose-ringed Parakeets
typically breed in Green Woodpecker cavities (with a diameter of approximately 6 cm) it
is difficult for an adult human to reach inside. Enlarging the cavity allows an adult to
reach inside and remove the chicks. However, this is not a suitable technique for
managing parakeet populations as it damages the tree in the process and is frowned upon
by park managers.
Efforts to manage parakeet numbers should probably try to manipulate parakeet
populations at the roost. In the UK, parakeets gather into a small number of communal
roosts throughout the year (n = 5), and these roosts can range in size from < 100
individuals to >5000 individuals (see Chapter 2). It has been demonstrated above that
adult survival rates are driving the exponential increase in Rose-ringed Parakeet
populations. Reducing those survival rates would slow the population growth or even
reverse it completely. However, roosts contain both adults and subadults, and it is
difficult to distinguish between adult females and subadult males. Therefore, it is more
logical to evaluate what level of harvest of the entire population is required in order to
slow or halt the parakeet population increase. According to figures 7 and 8, if managers
can harvest 30% of the total population annually, it is possible to reverse the population
growth exhibit by this species.
264

Management of feral parrot numbers in the short-term is possible, as evidenced by
the status of Monk Parakeets in the US. In 1967, the first free-flying Monk Parakeets
were reported in the US (Neidermyer and Hickey 1977, Nehls 1986). According to the
popular press, the numbers of Monk Parakeets in the US exploded to 4000-5000 birds by
1973 (Neidermyer and Hickey 1977). As Monk Parakeets are reported to be crop pests in
Argentina (and could potentially engage in interspecific competition with native species),
the US Fish and Wildlife Service coordinated a “retrieval” (i.e. an eradication) program
among 13 states where they had been reported (Neidermyer and Hickey 1977).
Neidermyer and Hickey (1977) conducted a study to estimate the number of
Monk Parakeets in the US after the eradication program began. They received reports of
367 birds during the period 1970-1975 and concluded that the estimates of 4000-5000
parakeets during the early 1970s had been vastly inflated. They confirmed that the
program to eradicate Monk Parakeets in the US was successful in reducing the numbers
of Monk Parakeets in the US, as nearly half of the reports they received of Monk
Parakeets were of birds that were removed from the wild (Neidermyer and Hickey 1977).
Since that time, however, Monk Parakeet numbers seem to have more than
recovered. Populations can now be found in Alabama, Connecticut, Delaware, Florida,
Illinois, Louisiana, New Jersey, New York, Oregon, Rhode Island, and Texas (Spreyer
and Bucher 1998). Numbers of Monk Parakeets in the US are increasing exponentially,
with the population doubling every 4.8 years (van Bael and Pruett-Jones 1996). The US
population must now consists of several thousand birds; during the 102 nd Christmas Bird
Count, 3015 individuals were counted in Florida and 908 individuals were counted in
Connecticut (Pranty 2002).
265

Efforts to manage Rose-ringed Parakeet numbers therefore will need to be carried
out on an annual basis in order to prevent a similar scenario. This species’ potential for
rapid population growth means that populations can recover in a very short time. In
addition, it’s continuing popularity as a pet means that continual introductions will
undoubtedly occur. Consequently, even if Rose-ringed Parakeet populations are
exterminated, it is probable that they will become established again in the UK.
In conclusion, Rose-ringed Parakeets populations are increasing very rapidly,
with the population in the Greater London area expected to consist of 10,000 individuals
by 2004. (During Aug 2003, an estimated 6900 parakeets were counted at the Esher roost
[Ross, pers. comm.] and during December 2003, there were an estimated 1750 parakeets
at the Lewisham roost [Hazlehurst, pers. comm.]. I have not received any information on
the sizes of the roosts at Maidenhead or Reigate during 2003). Populations are currently
established on 2300 km 2 in the Greater London area and 200 km 2 on the Isle of Thanet,
although their range expansion is progressing at a rate of only 0.4 km/yr. The rapid
population growth coupled with the slow rate of expansion will lead to high population
densities in areas where parakeets occur. As parakeets have been demonstrated to cause
damage to British crops, efforts to control the population size may be worthwhile.
Parakeets concentrate into a relatively few communal roosts throughout the year, and
harvesting 30% of the parakeets at these roosts annually will be sufficient to gradually
reduce their population. Whether such harvests will be eventually be carried out will
depend upon developing a method of harvesting that is suitable for public areas and
whether the members of the public who enjoy seeing free-flying parakeets in the UK can
be convinced that control of the Rose-ringed Parakeet population is necessary.
266

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273

TABLES:
Table 1: A summary of the Leslie matrices used in the population modelling. Rose-ringed
Parakeets in the Greater London area begin breeding by the age of 2, while Rose-ringed
Parakeets at the Isle of Thanet have not been observed breeding before the age of 3.
Monk
Parakeet
mortalities
Puerto
Rican
Parrot
mortalities
Model 1
Model 2
Greater London Isle of Thanet
⎡ 0
⎢
⎢
0.
61
⎢⎣
0
0
0
0.
81
1.
159⎤
0
⎥
⎥
0.
81 ⎥⎦
0 0 1.283 1.283 1.283
0.675 0 0 0 0
0 0.848 0 0 0
0 0 0.848 0 0
0 0 0 0.848 0.913
⎡ 0
⎢
⎢
0.
675
⎢⎣
0
⎡ 0
⎢
⎢
0.
65
⎢⎣
0
0
0
0.
775
0
0
0.
75
1.
283⎤
0
⎥
⎥
0.
875⎥⎦
1.
235⎤
0
⎥
⎥
0.
85 ⎥⎦
⎡ 0
⎢
⎢
0.
61
⎢ 0
⎢
⎣ 0
0
0
0.
81
0
0
0
0
0.
81
1.
16⎤
0
⎥
⎥
0 ⎥
⎥
0.
81⎦
0 0 0 1.28 1.28
0.675 0 0 0 0
0 0.848 0 0 0
0 0 0.848 0 0
0 0 0 0.848 0.913
⎡ 0
⎢
⎢
0.
675
⎢ 0
⎢
⎣ 0
⎡ 0
⎢
⎢
0.
65
⎢ 0
⎢
⎣ 0
0
0
0.
775
0
0
0
0.
75
0
0
0
0
0.
875
0
0
0
0.
85
1.
283⎤
0
⎥
⎥
0 ⎥
⎥
0.
875⎦
1.
235⎤
0
⎥
0
0.
85
⎥
⎥
⎥
⎦
274

Table 2: A comparison of life table figures for the two UK populations
Greater London Isle of Thanet
Long-term growth rate λ 1.325 1.170
Rate of increase r 0.281 0.157
Expected number of replacements R0 7.448 3.412
Generation time T 7.144 7.828
Mean age of reproducing adults µ 12 9.67
275

Table 3: Results from population wave formulas
Greater London Isle of Thanet
Observed breeding expansion rate 0.299 km/yr 0 km/yr
Observed sightings expansion rate 0.434 km/yr 0 km/yr
C ≈ 2rs
1.524 km/yr 1.139 km/yr
σ
C ≈ ln
µ
( 2 R )
o
σ ⎧ υ 2 1 ⎫
C ≈ ( 2ln
Ro
) ⎨1
+ [( ) − β+
γ ln Ro
⎬
µ ⎩ µ 12 ⎭
∞
n(x, t + 1) = ∫
−∞
∞
n(x, t + 1) = ∫
−∞
(Using α = 1.23)
[K(x-y)o Bn]n(y,t) dy
[K(x-y)o Bn]n(y,t) dy
0.443 km/yr 0.430 km/yr
1.876 km/yr 0.513 km/yr
2.303 km/yr 1.552 km/yr
0.685 km/yr 0.655 km/yr
276

FIGURES:
Figure 1: A chart of the log population growth for Rose-ringed Parakeets in the Greater
London area and the Isle of Thanet. The population in the Greater London area shows a
significant correlation with year (London Population = -233 + 0.118 * Year, F1,3 = 134.7,
R 2 = 97.8%, p = 0.001), as does the population at the Isle of Thanet (Thanet Population =
-97.3 + 0.05 * Year, F1,4 = 19.6, R 2 = 83.1%, p = 0.011). Note that the log population in
the Greater London is growing at more than twice the rate of the log population in the
Isle of Thanet.
Log of the population
4
3.8
3.6
3.4
3.2
3
2.8
2.6
2.4
2.2
2
Greater
London
Isle of
Thanet
1995 1996 1997 1998 1999 2000 2001 2002
Year
277

Figure 2: A chart of the population models for Greater London. Model 1 exhibits the
closest fit to the observed population growth. This model assumes 67.5% survival during
the first year, 77.5% survival during the second year, and 87.5% survival for each year
thereafter. See Table 1 for a summary of each model.
Population size
5400
4700
4000
3300
2600
1900
1997 1998 1999
Year
2000 2001
Puerto Rican
Parrot
Greater London
Model 1
Model 2
Monk Parakeet
278

Figure 3: A chart of the various models for the Isle of the Thanet. Model 2 shows the
closest fit to the observed population growth. This model assumes that Rose-ringed
Parakeets begin breeding at age 3 and that mortality is 65% during age 0, 75% during age
1, and 85% from age 2 on. See Table 1 for a summary of each model.
Population size
800
750
700
650
600
550
500
450
400
350
300
1997 1998 1999
Year
2000 2001
Puerto Rican
Parrot
Model 1
Isle of Thanet
Model 2
Monk Parakeet
279

Figure 4: A chart demonstrating the exponential growth potential of Rose-Ringed
Parakeets in the Greater London area. The population could exceed 10,000 by 2004 and
100,000 by 2036.
Population
10000000000
1000000000
100000000
10000000
1000000
100000
10000
1000
100
10
1
2001 2011 2021 2031 2041 2051
Year
280

Figure 5: A chart demonstrating the exponential growth potential of Rose-ringed
Parakeets at the Isle of Thanet. The population could exceed 10,000 by 2021 and 100,000
by 2036.
Population
10000000
1000000
100000
10000
1000
100
10
1
2001 2011 2021 2031 2041 2051
Year
281

Figure 6: Demographic sensitivities and elasticities for the Leslie matrices used to model
the population in the Greater London area and the Isle of Thanet area. In all cases, the
dominant value is the adult survival rate (the largest bar in the graph).
Sensitivites
Elasticties
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0
Greater London
1
Ages
2
3
Greater London
1
Ages
2
3
Sensitivities
Elasticities
0.6
0.5
0.4
0.3
0.2
0.1
0
0.4
0.3
0.2
0.1
0
Isle of Thanet
1 2 3 4
Ages
Isle of Thanet
1 2 3 4
Ages
282

Figure 7: A graph of the projected effect of six management regimes on the population
size of Rose-ringed Parakeets in the Greater London area. Parakeet populations are
projected to increase unless a management regime harvests 30+% of the population each
year.
Population size
10000000
1000000
100000
10000
1000
100
10
1
2000
2005
2010
2015
2020
2025
2030
Year
No management
10% mortality
(adults)
10% mortality (all)
30% mortality
(adults)
50% mortality
(adults)
30% mortality (all)
50% mortality (all)
283

Figure 8: A graph of the projected effect of six management regimes on the population
size of Rose-ringed Parakeets in the Isle of Thanet area. Parakeet populations are
projected to increase unless a management regime harvests 30+% of the population each
year.
Population
100000
10000
1000
100
10
1
2000
2005
2010
2015
2020
2025
2030
Year
No Management
10% mortality
(adults)
10% mortality (all)
30 % mortality
(adults)
50% mortality
(adults)
30% mortality (all)
50% mortality (all)
284

Figure 9: A map of the data (courtesy of the BTO) included in the 1988-91 Breeding Bird
Atlas (Gibbons et al. 1993). This map is organized into 10 km x 10 km squares (100
km 2 ). Squares where Rose-ringed Parakeets were confirmed as breeders, as well as
sightings during 1988-91 are shown.
285

Figure 10: A map of sightings where parakeets were found breeding or were seen
repeatedly during 2000-03.
286

Figure 11: A graph of the number of number of squares occupied over time. A linear
relationship is apparent, but is only very weakly significant ( Area = -2046 + 1.05 *
Year, F1,2 = 16.20, R 2 = 89.0%, p = 0.057).
Square root of the area occupied
60
50
40
30
20
10
0
1970 1975 1980 1985 1990 1995 2000 2005
Year
287

Figure 12: A histogram of dispersal distances (α) based on new breeding records from
county bird reports in the Greater London area.
288

Figure 13: Wavefront sensitivities and elasticities for the Greater London and Isle of
Thanet populations. For both locations, the element that has the greatest effect on
wavefront speed is adult survival (the largest bar).
Wavespeed
sensitivites
Wavefront
elasticities
Greater London
2.5
2
1.5
1
0.5
0
0.4
0.3
0.2
0.1
Ages
1 2 3
Greater London
0
1
Ages
2
3
Wavefront
sensitivities
0.4
Isle of Thanet
0.3
0.2
0.1
Wavefront
elasticities
0
1 2 3 4
Ages
Isle of Thanet
0.4
0.3
0.2
0.1
0
1 2 3 4
Ages
289

Chapter 8:
Concluding comments
A number of theories have been advanced to explain why some introductions
succeed while others fail (see Chapter 3). Broadly speaking, they can be broken into two
categories; intrinsic and extrinsic. Intrinsic theories suggest that certain life-history
characteristics and morphological traits predispose an alien species to becoming
successfully established (e.g. high fecundity, low demographic stochasticity, etc.; see
Lockwood 1999, Duncan and Blackburn 2002). Extrinsic theories suggest that habitat
qualities (e.g biotic resistance, area, etc.; see Crawley 1986, Case 1996) are more
important.
Rose-ringed Parakeets are the most widely introduced parrot species in the world
(Long 1981, Lever 1987, Forshaw 1989), and consequently provide an excellent system
for testing both intrinsic and extrinsic hypotheses. A discriminant function was created to
separate countries where they have become naturalized from countries where they failed
to establish self-sustaining populations (see Chapter 3). The function was successful in
predicting establishment success based upon both intrinsic and extrinsic factors. This
suggests that future studies may find it useful to incorporate both intrinsic and extrinsic
factors to examine patterns of successful introductions.
Although Rose-ringed Parakeets have been introduced into 35 countries (see
Chapter 3), little work has been done on their biology in countries where they have been
introduced. For example, although these parakeets are breeding in northern countries
290

such as the UK, Germany, and the Netherlands, it is unknown if they are reliant upon
feeders in order to survive during the winter. At least one author has suggested that Rose-
ringed Parakeets in the UK do not need feeders in order to survive, as a population of
parakeets in Brighton apparently did not learn how to use feeders for eight years (James
1996). I was unable to address this issue in my study, as it was impossible to determine
what parakeets were feeding upon based on fecal samples, as the samples were a uniform
paste. However, Rose-ringed Parakeets are routinely seen in rural areas of Berkshire,
Buckinghamshire, Surrey and Sussex, suggesting that they are not wholly dependent
upon suburbia. In addition, I have observed parakeets feeding upon buds, seeds, and
mistletoe during the winter, suggesting that they are able to utilize other food sources in
addition to feeders.
However, it should be noted that, unlike native species, Rose-ringed Parakeets do
not gain weight during the winter (see Chapter 6). This may mean that during prolonged
periods of inclement weather, they could suffer increased mortality. During weather
events such as these, feeders may be more important than they are during most of the
winter.
Regardless of whether Rose-ringed Parakeets are reliant upon feeders, their
numbers have increased since they were first introduced into England. Feral Rose-ringed
Parakeets began breeding annually in the UK in 1969 (Lever 1977) and by 1983 the
British Ornithologists’ Union accepted these parakeets as a Category C (established
exotic) species and estimated the population to consist of approximately 500 individuals
(BOU 1983). By 1986, it was estimated that there were 500-1000 parakeets in the UK
(Lack 1986). It was not until 1996, however, that the first census was carried out (Pithon
291

and Dytham 1999). This census surveyed all the known roosts during the winter and
concluded that there were approximately 1500 parakeets in the UK (Pithon and Dytham
1999a). These authors concluded that the population of Rose-ringed Parakeets was
increasing very slowly, based on the difference between the high end of Lack’s (1986)
estimate (e.g. 1000 birds) and their census (1500 birds).
However, when I began my research in 2000, it soon became clear that Rose-
ringed Parakeet populations were increasing more rapidly than previously thought.
During the winter of 2000/2001, a total of approximately 4200 parakeets were detected at
the four known roosts. During the winter of 2001/2002, a new roost of approximately 120
parakeets was found, bringing the total of to 5800 parakeets in the UK (see Chapter 2).
From 1996 to 2002, Rose-ringed Parakeets were increasing at a rate of approximately
25% per year.
Whether the rate of increase has truly increased during the past few years is open
to speculation. It is possible that this apparent rapid rate of increase recently may be an
artifact due to an overestimation of the parakeet population during the 1980s. Rose-
ringed Parakeets are fairly noisy, visible birds, and it is possible that the numbers
reported during the 1980s may have been overinflated. Potentially, it is possible that a
relatively small population of Rose-ringed Parakeets may have been increasing at a rate
of 20+% per year since the 1980s.
It soon became apparent that parakeet reproductive success in the UK must be
higher than previously thought. On the Indian subcontinent, Rose-ringed Parakeets
fledged 1.7 – 3.0 young per nest (Lamba 1966, Shivanarayan et al. 1981). However, a
study on the breeding biology of this species in the UK during 1997-1998 concluded that
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parakeets in England fledged only 0.8 young per nest (Pithon and Dytham 1999b). For
the period of 2001-2003, I located and monitored 108 Rose-ringed Parakeet nests in
Greater London and the Isle of Thanet and found the parakeets fledged 1.9 ± 0.1 young
per nest (see Chapter 4).
This relatively high rate of fecundity in the UK is consistent with the observed
population explosion of Rose-ringed Parakeets. However, the Greater London population
is growing faster than the Isle of Thanet population: parakeet populations in Greater
London are growing at a rate of 30% per year, while populations at the Isle of Thanet are
increasing at a rate of only 15% per year (see Chapter 7).
Introduced species can have many negative impacts (see Chapter 1). They can
compete with native species for limiting resources (Pell and Tidemann 1997), predate
native species (Savidge 1987), alter habitats (Daehler and Gordon 1997, Mack and
D’Antonio 1998), introduce diseases (Pratt 1994), hybridize with native species
(Manchester and Bullock 2000), and cause economic damage (Fritts 2002).
Initially, it was believed that Rose-ringed Parakeets introduced into England
might cause economic and ecological damage. Lever (1977) speculated that Rose-ringed
Parakeets could potentially outcompete native cavity-nesting species for nest sites. In
addition, Rose-ringed Parakeets have been reported to cause considerable damage to
agriculture on the Indian subcontinent (Lever 1987, Long 1981, Mukerjee et al. 2000,
Reddy 1998a, Reddy 1998b), and so they could potentially have a detrimental effect on
British agriculture as well. Obviously if parakeet populations are rapidly increasing, there
is the potential for Rose-ringed Parakeets to outcompete native cavity-nesting species for
nest sites as Lever (1977) suggested. However, to date, they do not appear to be having a
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negative impact on native species. In 2001, a summary of the population changes
between 1994 and 2000 in species surveyed by the Breeding Bird Survey (BBS) was
released (Noble et al. 2001). Although some species exhibited significant (e.g. European
Starling) or nonsignificant (e.g. Eurasian Kestrel and Stock Dove) declines, Rose-ringed
Parakeets were not listed in this report, presumably because they did not fit the BTO’s
criteria, i.e. that the species be “recorded in at least 20 squares in 2000 in two or more of
the nine Regional Development Agency (RDA) regions of England” (Noble et al. 2001).
Despite the fact that parakeets have increased from approximately 1500 individuals in
1996 (Pithon and Dytham 1999a) to 5800 individuals by 2002, neither the BBS nor the
CBC (Common Bird Census) plots have enough breeding parakeets to track changes in
the population (J. Marchant, pers. comm.). This suggests that parakeets are unlikely to be
causing the declines observed in Kestrels, Stock Doves, and Starlings in southeastern
England (which are declining in parts of the country where Rose-ringed Parakeets do not
occur). Their propensity for using Green Woodpecker cavities, however, may potentially
limit the spread of the population north, as Green Woodpecker are relatively uncommon
in Scotland.
Although Rose-ringed Parakeets will nest in rock crevices and buildings in their
native range (Ali and Ripley 1969, Roberts 1991, Juniper and Parr 1998), all the nests
found both in this study and those reported by Pithon and Dytham (1999b) were in trees,
suggesting that rock crevices and buildings are not used in the UK. If so, this may reduce
the competition with native species as Kestrels, Stock Doves, Jackdaws, and Starlings. In
addition, Rose-ringed Parakeets defend only their nest cavity rather than the entire tree, a
result consistent with that observed in India (Lamba 1966). During 2001-2003, parakeets
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were observed sharing their nesting tree with European Starlings, Stock Dove, and
Jackdaw (see Chapter 4).
The apparent high rate of fecundity that I discovered may eventually be a
problem. If parakeets continue to increase at a rate of approximately 25% per year, then it
is likely that nest cavities may become a limiting factor in the future, even though
parakeets defend only their nest cavity rather than the entire tree. Since Rose-ringed
Parakeets begin nesting in late February, the potential still exists for them to outcompete
native cavity-nesting species in the future.
To date, Rose-ringed Parakeets have apparently caused little ecological damage.
However, there is evidence that they are beginning to adversely affect crops. An article
carried in the Kingston Guardian in October 2002 describes the damage that parakeets
caused to a vineyard in Painshill Park (Surrey) (Saines 2002). Traditionally, this vineyard
produces 3000 bottles of rosé wine for consumption. However, in 2002 the parakeets
decimated the grape crop, and only 500 bottles could be produced (Saines 2002).
Although Rose-ringed Parakeets are spreading into the countryside, the bulk of
their population is still centred in urban areas. In Chapter 7, models of population growth
and spatial spread were evaluated. Although populations of parakeets in the Greater
London area are increasing at a rate of 30% per year, they are only expanding their range
at a rate of 0.4 km per year. This situation will lead to a high density of parakeets in areas
where they occur. The prospect of high parakeet densities may be not be a welcome one
to farmers in Surrey, Middlesex, Berkshire, Buckinghamshire and Kent as Rose-ringed
Parakeets have already demonstrated that they can have a negative impact on crops in
Britain.
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In order to prevent the Greater London population from increasing further, it will
be necessary to harvest 30% of the population annually. The easiest way to do this would
be to harvest parakeets when they come into a roost. However, as most of these roosts are
located in suburban areas (e.g. a rugby club, a cemetery, etc.) the high visibility of such
an effort will need to be considered.
In the short-term, a reduction in parakeet numbers is certainly achievable. A
concerted effort by the US during the 1970s succeeded in reducing feral Monk Parakeet
numbers in half (Neidermyer and Hickley 1977). However, Monk Parakeets are popular
pets, and once the eradication efforts ceased, their populations began to recover. Current
populations now exceed the 1970s population estimates. Neidermyer and Hickley (1977)
estimated that there were only a few hundred feral Monk Parakeets during the 1970s. In
contrast, the US population must now consist of several thousand birds; during the 102 nd
Christmas Bird Count, 3015 individuals were counted in Florida and 908 individuals
were counted in Connecticut (Pranty 2002). It is certainly reasonable to suppose that any
effort to reduce Rose-ringed Parakeets might face the same problem; because Rose-
ringed Parakeets are popular pets, it is likely that releases will occur annually, and
numbers of feral parakeets may grow again unless repeated efforts are made to reduce the
population.
Whether the cost of pursuing a reduction in Rose-ringed Parakeet numbers is
worthwhile is debatable. Rose-ringed Parakeets are afforded legal protection by the
Wildlife and Countryside Act 1981, and they are not thought to be affecting any
endangered species (unlike the introduced Ruddy Duck Oxyura jamaicensis which
hybridizes with the endangered White-headed Duck Oxyura leucocephala and which is
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currently being harvested in the UK in order to reduce its population). Indeed, Rose-
ringed Parakeets do not appear to be having a negative impact on any native cavity-
nesting species, although this could change as their population continues to grow.
Although they have been reported to cause damage to crops in Britain, these reports are
few and far between (e.g. Saines 2002). Given that there are several thousand parakeets
in the UK, the lack of reported crop damages suggests that their potential as a crop pest in
England may be limited. Although there has been concern that feral parrots may cause
crop damage in countries where they have been introduced (e.g. State of California 1976,
Neidermyer and Hickley 1977), to date these fears of widespread crop damage have
failed to materialize. Consequently, it might be more cost-effective to allow farmers
whose crops are being depredated to reduce parakeet numbers on their farms, rather than
initiating a national programme aimed at reducing feral parakeet numbers.
In the future, it would be worthwhile to conduct annual censuses on the roosts in
order to better track changes in the population. It would also be worthwhile to begin a
long-term study on the edge of the parakeets’ current range to examine what happens to
native cavity-nesting species when Rose-ringed Parakeets invade. Future studies on the
movements of individual birds using radiotelemetry (perhaps utilizing a reinforced radio
transmitter) would also shed light on how far individuals are willing to travel. A
quantitative study of their food habits would help answer whether these parakeets are
dependent upon feeders in the UK. In addition, long-term mark-recapture studies would
also be useful to help establish the age structure of the population and mortality rates.
In conclusion, Rose-ringed Parakeets are a widespread, successful invasive exotic
species with a firmly established population in the UK. Although initial population
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growth was slow, parakeet populations in the UK are now growing at a rate of
approximately 25% per year. Reproductive rates in England are much higher than
previously reported in the literature. Despite this rapid increase in numbers (the
population will exceed 10,000 individuals by 2004) there is no evidence that feral
parakeets are having a negative impact on native species, and there have been relatively
few reports of crop damage. Consequently, whether the benefit of reducing the
population of feral parakeets in the UK outweighs the costs of such a program should be
carefully examined. Certainly, Rose-ringed Parakeets are not the scourge on the British
countryside that the media often portrays them as.
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302

Appendix 1
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Appendix 2
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Appendix 3
List of published papers
Butler, C. 2003. The disproportionate effect of climate change on the arrival dates of
short-distance migrant birds. Ibis 145: 484-495.
Butler, C. 2003. Hooded Merganser, Arctic Tern, California Gull, Ring-billed Gull. In:
Marshall, D. B., M. G. Hunter, and A. L. Contreras (eds.) Birds of Oregon: a general
reference. Corvallis, OSU Press.
Butler, C. 2003. Rose-ringed Parakeets in the UK. PsittaScene 15: 8-9.
Butler, C. 2003. A brief summary of some results of my PhD research into “The
population biology of the introduced Rose-ringed Parakeet Psitacula krameri in the UK”.
Runnymede and Maple Cross Ringing Groups Annual Report 2002: 56-58.
Butler, C. 2002. Breeding parrots in Britain. British Birds 95: 345-348.
Butler, C., Hazlehurst, G., & Butler, K. 2002. First nesting by Blue-crowned Parakeet in
Britain. British Birds 95: 17-20.
Butler, C. 2002. Space invaders. The newsletter: the magazine of Bolton RSPB Members'
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Butler, C. 2001. Project Parakeet. Cheltenham Bird Club Winter 2001: 24-27.
Butler, C. (in press). Monk Parakeets in Oregon: Species Status Review. Oregon Birds.
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