monitoring breeding birds in the united kingdom - European Bird ...

R. GREGORY, S. BAILLIE & R. BASHFORD , 2004 - Monitoring breeding birds in the United Kingdom. In: Anselin, A. (ed.) Bird
Numbers 1995, Proceedings of the International Conference and 13 th Meeting of the European Bird Census Council,
Pärnu, Estonia. Bird Census News 13 (2000):101-112
MONITORING BREEDING BIRDS IN THE UNITED KINGDOM
R. D. Gregory 1,2 , S. R. Baillie 1 & R.I. Bashford 1,2
ABSTRACT. A new annual survey of terrestrial breeding birds was introduced
in the United Kingdom (UK) in 1994. The aim of the scheme, known as the
Breeding Bird Survey (BBS), is to monitor population levels of a broad
spectrum of common and widespread species across a representative sample of
sites and habitats in the UK. Population changes will be interpreted in the light
of habitat and other environmental data sets. A primary objective of the scheme
is to identify species of conservation concern and, in combination with other
data from the British Trust for Ornithology's Integrated Population Monitoring
Programme, to provide pointers to the causes of population changes. Survey
squares, 1 × 1 km squares of the UK National Grid, are chosen as a random
sample stratified by potential observer density. Birds are counted twice during
the breeding season using standardised line transects. Birds are recorded in three
distance categories from the transect line or as in flight. Habitat data are
recorded annually for each 200 m section of transect. In the first year of the
scheme, 1994, over 1 500 squares were surveyed and 190 species recorded. A
total of 76 species were recorded from over 100 separate squares. We aim for
the sample size to rise to 2-3 000 squares over the next few years. Here we
describe the background to the introduction of the BBS, details of survey and
methods, and preliminary results from 1994.
1. British Trust for Ornithology, The Nunnery, Thetford, Norfolk
IP24 2PU, United Kingdom.
2. Current address: The Royal Society for the Protection of Birds, The Lodge,
Sandy, Bedfordshire, SG19 2DL United Kingdom.
INTRODUCTION
The need to monitor wildlife populations has arguably never been so great with rapid
development, urbanisation and industrialisation across the globe. Effective bird conservation is not
possible without monitoring information to tell us how population levels are changing, and ideally,
providing pointers as to why these changes are taking place. The monitoring of bird populations has
the added advantage that birds can act as valuable barometers or indicators of the general health of
the countryside (Furness & Greenwood 1993).
Large-scale surveillance of birds in the UK is made possible by the number of skilled and
dedicated ornithologists who are willing to participate in survey work. Compared with other taxa,
birds are relatively easy to detect and identify, particularly in the breeding season. The British Trust
for Ornithology (BTO) has a long history of involvement in monitoring and associated research. It
is the unique partnership between skilled volunteers and the professional staff that sets their
research programme apart from other organisations in the UK. The BTO works closely with other
organisations concerned with the conservation of birds, particularly by the statutory conservation
bodies for the UK, the Joint Nature Conerservation Committee (on behalf of the country agensies),
and the Royal Society for the Protection of Birds (RSPB). The increasing collaboration among
organisations reflects mutual interest and concern for the fate of bird populations in the countryside.
The statutory requirement to monitor birds arises in law from the Wildlife and Countryside Act
1981, the European Union Birds Directive, and the Ramsar, Bonn and Berne Conventions.
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The scope of monitoring
A total of 215 bird species breed in the UK on a regular basis (Gibbons et al. 1993). Over 80 %
of those species are censused or surveyed on an annual basis through a variety of surveys supported
by government and non-government conservation organisations (Fig. 1: Greenwood et al. 1995).
Specific surveys cover population changes of the Heron, wetland birds (10 species), seabirds (22
species), and rare breeding birds (56 species: being defined as having populations of fewer than 300
pairs). We envisage that a small number of birds (27 species) can be covered by improved collation
of data collected by bird clubs, reserve wardens and raptor monitoring groups (Fig. 1). There is a
group of ten species for which there is no specific survey at present.
Figure 1. Monitoring of terrestrial breeding birds in the United Kingdom.
The graph shows the approximate number of species monitored by various annual surveys
and recording groups. The proposed monitoring of species covered by raptor and owls
monitoring, and improved data collation, have yet to be implemented. Data from autumn
Wetland Bird Survey counts would be used to monitor a small number of breeding
waterfowl which are not covered by other schemes.
The remaining breeding birds (89 species) represent the common and widerspread species of
the countryside and these form the focus of BTO's annual monitoring programme. At this scale, it is
not feasible nor efficient to count or estimate total population sizes as it is for the rarer species, and
monitoring must be based on sample surveys. For the information to be of greatest use, it is crucial
for the sample of sites to be representative of the countryside as a whole. The primary objective of
bird monitoring, or more strictly surveillance, is to provide an index of population changes from
year to year. Although the terms monitoring and surveillance tend to be interchanged in the
literature, monitoring implies (1) the measurement of changes against predefined standards of
targets, (2) the collection of supplementary data to help intepret and understand population changes,
and (3) that there are clear objectives (Baillie 1990, Furness & Greenwood 1993).
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COMMON BIRD CENSUS
The Common Bird Census (CBC) has been the main monitoring tool for common birds in the
UK over the last 30 years. The CBC is based on territory mapping and is one of the longest running
schemes of its kind in Europe, having been established in the early 1960s (Marchant et al. 1990). It
has proved highly valuable in advancing our knowledge and understanding of population
fluctuations among British birds (e.g. O'Connor & Shrubb 1986, Marchant et al. 1990, Baillie 1990,
Greenwood & Baillie 1991, Peach et al. 1991, Fuller et al. 1995, Greenwood et al. 1995). There is
general consensus that territory mapping provides a reasonable estimate of most territorial
populations (Dawson 1981, Verner 1985, Verner & Ritter 1988, Bibby et al. 1992) and the aims
and methods of the CBC have been reviewed on a periodic basis (O'Connor & Fuller 1984,
Marchant et al. 1990, Baillie & Marchant 1992).
Despite the considerable achievement of the CBC, the scheme has a number of limitations and
the BTO has been exploring alternative methods for population monitoring for some time (Baillie &
Marchant 1992, Gregory et al. 1994). The limitations of the CBC are as follows:
1. Survey sites are not chosen at random. In consequence, there is the potential for species,
geographical and habitat bias. In fact, the CBC has been restricted to farmland and
woodland, and other habitats have been excluded. The distribution of survey plots mirrors
the distribution of the human population and is baised towards the south and east of the
UK.
2. Fieldwork, data compilation and mapping analysis are extremely time consuming (Gregory
et al. 1994). Fieldwork involves nine or ten visits to a census plot each breeding season and
many hours in data collation. Note that in Britain territory analysis is carried out by BTO
staff rather than by volunteers, as is the case in other countries (van Dijk 1992). The time
required for territory mapping fieldwork and analysis is roughly seven times that of an
equivalent technique, such as point counts (Gregory et al. 1994).
Taken together, these factors have limited the expansion of CBC coverage in the UK and to
some extent limited the use of the data, although much can be done at the analytical stage to
alleviate problems (Greenwood et al. 1995). Over a number of years the BTO has tested a number
of new approaches to population monitoring in a UK setting (Gregory & Baillie 2004). The aim is
to develop a monitoring scheme based on a formal sampling strategy for volunteer birdwatchers
which uses simple and efficient methods and thus allows large numbers of people to participate
over a wide geographical area.
ALTERNATIVES TO TERRITORY MAPPING
Line and point-count transects
The obvious alternatives to territory mapping are line or point count transects, both of which
are relatively efficient and have been used widely in population monitoring in Europe and North
America (Bibby et al. 1992). A number of field trials have tested the suitability of these methods
(Gregory et al. 1994, Gregory & Baillie 2004). They showed the comparability of monitoring
results derived from different counting methods, the efficiency of the methods, and the
practicability of using randomly chosen survey sites in the UK. A line transect approach was
recommended because it was preferred by volunteers and was slightly more precise. Furthermore,
line transects can be applied effetively in a wider range of habitats than point counts (Bibby
et al. 1992).
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The final, and arguably most important, aspect of the new scheme was the formal sampling
strategy. Our trials, and similar experience with national surveys of single-species (Shrubb & Lack
1991, Donald & Evans 1995), have demonstrated the feasibility of a random sampling design in the
UK. The advantages of this are considerable as it provides the only mechanism for producing what
is demonstably unbiased and thus representative information (Bibby et al. 1992). A desk-based
study assessed a number of different strategies for the selection of survey squares (Gregory &
Baillie 1994, 2004). These simulations showed that simple random sampling compared very well
with any of the more complicated strategies involving landscape type. It was concluded that a
random selection, stratified by potential observer density, should be used for the new survey.
BREEDING BIRD SURVEY
Against this background, an annual survey of terrestrial breeding birds was introduced in the
UK in 1994 (http://www.bto.org/survey/bbs.htm). The strategic aims of the BBS are:
1) to be based on a formal randomised sampling design,
2) to improve the geographical representation of monitoring,
3) to improve the habitat representation of monitoring, and
4) to increase species coverage of monitoring (as a consequence of the other points).
The BBS thus aims to provide precise information on year-to-year and longer term changes in
population levels for a broad spectrum of common species across a range of regions and habitats.
The parallel recording of land use and habitat change will facilitate a better understanding of the
factors responsible for population changes, and this will be particularly important for populations in
decline. In a wider context, the BBS aims to promote a greater understanding of the population
biology of British birds through a partnership between large numbers of voluteer and a small
number of professional ornithologists.
Selection of survey squares
Survey squares were selected as a random sample, stratified by potential observer density,
from within regions across the UK (Gregory & Baillie 1994). The 128 BTO regions were reduced
to 83 regions by the amalgamation of the smallest ones. Sampling from fewer, larger regions avoids
the problem of extremely small administrative regions. The number of squares to be surveyed in
each region was calculated by multiplying observer desity by a constant and setting a minimum
level of coverage. We anticipate that professional will be necessary to ensure minimum coverage in
remote areas.
At a local level squares are allocated to volunteers through a network of voluntary regional
organisers. Each organiser receives a list of target squares for their region with the instruction that
squares should be allocated in strict order from the top downwards. This is essential in maintaining
the random design of the survey. The same squares are surveyed year after year and new volunteers
are found if the original volunteer drops out. The way in which observer changes are handled in the
analysis will be addressed once sufficient data have been gathered. Regional organisers play a vital
role in coordinating fieldwork and overseeing the return of data. In limited circumstances, they can
report squares as being uncoverable, for example, because a landowner refuses access permission or
the square is complelely inaccessible. In such cases, the square can effectively be ignored, although
organisers submit information on land use and habitat type to check for any bias introduced by their
exclusion.
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Survey methods
BBS fieldwork involves three visits to a survey square each year. The first visit is to record
details of the habitat and to mark the survey route (Fig. 2). Bird counts are carried out on the second
and third visits. The survey route comprises two parallel lines, each 1 km in length (although for
practical reasons there is often substantial deviation about the ideal). Each of these lines is divided
into five sections, making a total of ten 200 m sections, and birds and habitats are recorded within
these units. Habitat type and land use are recorded annually on a habitat form. This form describes
both the habitat surveyed along the actual and the 'ideal' transect if there is a deviation. By recording
the 'ideal' transect route we are able to access whether observers avoid or prefer particular habitats
within their squares.
Figure 2. An idealised BBS route through a 1 × 1 km square.
The lower drawings show proposed survey routes through 1 × 1 km squares.
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All the survey forms were designed so that the data could be readily computerised. For
example, coded information on species names, county codes, weather conditions, and habitat type
allow detailed information to be summarised and input efficiently. Habitat information is recorded
on the first reconnaissance visit to the square. The appropriate habitat codes are chosen from an
established hierarchical system (Crick 1992), which is common to a range of BTO schemes.
Observers record the primary and secondary habitat for each transect section with up to four levels
of detail (Fig. 3). The primary habitat is the dominant habitat type within a particular transect
section. In the example given, transect 1 comprises tilled land with a hedgerow without trees, an
active farmyard, and the dominant crop being an autumn cereal. There is no secondary habitat
within the section and so this space is left blank. Note that the ideal transect lies within 20 m of the
actual route and the habitats do not differ (Fig. 2). Transect 2 is a similar area of farmland which
contains a small woodland. The first habitat codes are the same as those above. The second codes
are for a young coniferous plantation with low human disturbance, a moderate shrub layer, and
sparse field layer. Note that the ideal route is 50 m from the actual survey route and the ideal goes
through the young plantation.
Figure 3.
An example of a BBS habitat recording form (see text for details).
Count visits are timed so that the first is in the early part of the breeding season (April to mid-
May) and the second in the late part (mid-May to the end of June). It is recommended that visits
should be at least four weeks apart. The average visit time is around 90 minutes. Volunteer counters
are asked to begin their counts between 6 am and 7 am so that they coincide with maximum bird
activity, but avoid concentrated song activity at dawn (Bibby et al. 1992). Volunteers record all the
birds they see or hear as they walk methodically along their transect routes. They are encouraged to
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pause, listen and scan for birds as they walk along their transects. Birds are noted in three distance
categories (within 25m, between 25-100 m, or over 100 m to either side of the line) measured at
right angles to the transect line, or as in flight. Recording birds in distance bands is important
because it gives a measure of bird detectability in different habitats and allows relative population
density to be estimated using DISTANCE sampling (Bibby et al. 1992, Buckland et al. 1993). Note
that the estimates of absolute densities derived using this method are based on a number of key
assumptions.
Birds are noted in the field on specially designed forms using two-letter species-codes, which
were developed within the CBC (Fig. 4). Observers are encouraged to summarise this information
on count summary forms as soon after the fieldwork as is convenient. Header information on the
forms includes the observers' name, address, telephone number, the square reference, county code,
date, whether the bird count was early or late in the season, weather conditions, and the starting and
finishing times of the two halves of the transect (Fig. 4). Counties are recorded using 4-letter codes
which are used across BTO schemes. Weather codes describe cloud cover, rain, wind, and visibility,
on arbitrary three-point scale. We discourage bird counts in heavy rain, poor visibility, or strong
winds.
Figure 4. An example of a BBS field recording form and summary sheet (see text for details).
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The majority of forms are returned to BTO Headquarters through our network of regional
organisers. Thus organisers are in a position to chase up outstanding forms, answer queries and note
interesting observations. BTO acknowledges the receipt of each form directly with each observer
and we provide them with information on census work and related projects on a regular basis. On
receipt, count summary and habitat forms are double-checked by staff for clarity and obvious errors,
before being sent to be input by an outside agency. Field forms are kept for reference and to allow
any further checking. A series of computer progams are used to check the computerised data set.
These programs look for inconsistency or errors in the different codes (a number of similar species
codes are commonly confused) and check for unlikely bird counts or occurences.
Results from the first year
Coordinates for a total of 2 434 1 × 1 km squares were distributed to regional organisers in
1994. 1 553 (64 %) of these squares were surveyed (Fig. 5), 734 (30 %) were not surveyed and 147
(6 %) were reported as uncoverable. A small number of squares 68 (4 %) were surveyed by
professional teams in north and west Scotland funded by RSPB. This is an area with very few
volunteer counters. The map shows an encouraging spread of survey squares although there are a
number of gaps which we intend to fill in the coming years. The priority in 1995 has been to
improve the proportion of squares that are surveyed and only those regions achieving a high level of
coverage in 1994 received any new squares for 1995. The eventual sample size of BBS is to be
2 000-3 000 1 km² squares.
Figure 5. Distribution of Breeding Bird Survey squares in 1994.
The sampling strategy is designed so that there are larger numbers of squares where there
are more potential observers and the map shows a concentration of points around large
cities.
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The range of species recorded in 1994 was extremely broad, the total species count was 190,
with 76 species recorded from over 100 squares, and 21 species from between 50-100 squares
(Table 1). This suggests that we would be able to monitor these species with at least a moderate
degree of precision. The most widespread species were Chaffinch Fringilla coelebs, Woodpigeon
Columba palumbus, Blackbird Turdus merula, and Wren Troglodytes troglodytes. A wide range of
habitats was covered (Table 2). Farmland predominates within the sample, as it dominates land use
in the UK (Barr et al. 1993), the next most common land uses were human sites (including urban,
suburban and rural sites) and woodland, followed a range of rarer land types (Table 2). We expect
the coverage of species and habitats to increase modestly as the scheme grows.
DISCUSSION
The launch of the BBS as an annual bird monitoring scheme marks an important and
significant advance in British ornithology. In the medium- to long-term the BBS will take over the
monitoring role of the CBC by producing population indices for a range of common breeding birds.
Calibration of the trends from the two schemes will be essential to maintain the long-term time
series initiated by the CBC in the 1960s. We plan to have a considerable period of overlap between
the two surveys. It will be some time before there are sufficient BBS data to produce meaningful
time trends and in the meantime data from the CBC will be invaluable source of information.
Further work is required in the development of the analysis for year-to-year monitoring data
from stratified BBS samples, and longer-term population changes. The national statistics will be
derived from a region-by-region analysis. The fact that different regions have different sampling
intensities can be incorporated in the analysis and will not bias estimates of change. Population
changes are likely to be weighted by the size of regions, sampling intensity within regions, and the
number of bird registrations within occupied squares. We will explore the use of bird counts in
different distance categories and the use of early and late counts on a species-by-species basis. For
example, it might be sensible to treat residents, early-arriving migrants and late-arriving migrants in
different ways. In this way, it should be possible to maximise the precision of between-year changes
for groups of species with particular lifestyles. Recent advances in the analysis of monitoring data,
particularly the implementation of methods based on the Loglinear Poisson regression (ter Braak et
al. 1994, van Strein et al. 2004) provides great potential for use with BBS data. It is the nature of
such time-series that the data contain missing values, are spatially auto-correlated, and that the bird
counts will be non-normally distributed. All these factors, however, can be assessed and
incorporated in the regression models developed by van Strein et al. (2004) to create robust indicies.
BBS data will form an integral part of the BTO's Integrated Population Monitoring (IPM)
programme by providing information on populatin changes among common birds. The IPM aims to
develop a better understanding of bird populations by integrating information on bird numbers with
that on survival and breeding success (Baillie 1990, Greenwood et al. 1993). It is then possible to
build population models to explore the dynamical behaviour of a particular species and build in
environmental variables such as weather, in order to distinguish natural population fluctuations
from those brought about by the actions of man.
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Table 1. Species recorded from the Breeding Bird Survey 1994.
Species are listed in descending order of occurence for A) species recorded from over 100
survey squares and B) species recorded from between 50 and 100 squares.
Species
Proportion
of survey
squares
occupied
(%)
Number
of survey
squares
occupied
A. Over 100 squares occupied
Chaffinch 87 1349
Woodpigeon 87 1342
Blackbird 86 1340
Wren 86 1334
Carrion Crow 83 1290
Robin 82 1265
Blue Tit 79 1220
Starling 73 1124
Skylark 72 1113
Great Tit 71 1102
Dunnock 69 1076
Swallow 68 1048
Song Thrush 66 1023
Magpie 65 1014
Willow Warbler 62 964
House Sparrow 60 924
Greenfinch 59 909
Pheasant 58 894
Jackdaw 54 835
Yellowhammer 52 806
Linnet 51 785
Goldfinch 50 769
Rook 49 767
Collared Dove 46 716
Swift 46 710
Blackcap 46 707
Mistle Thrush 46 706
Whitethroat 43 667
Pied Wagtail 43 659
Mallard 42 656
Cuckoo 42 652
Chiffchaff 41 640
House Martin 34 524
Meadow Pipit 33 515
Lapwing 31 487
Long-tailed Tit 30 471
Kestrel 29 453
Stock Dove 29 451
Jay 27 419
Coal Tit 27 411
Black-headed Gull 24 369
Moorhen 24 368
Bullfinch 24 366
Great Spotted Woodpecker 23 362
Green Woodpecker 23 358
Feral Pigeon 23 358
Curlew 23 350
Goldcrest 22 346
Grey Heron 22 337
Species
Proportion
of survey
squares
occupied
(%)
Number
of survey
squares
occupied
contd.
Herring Gull 20 317
Lesser Black-backed Gull 19 292
Buzzard 18 284
Reed Bunting 17 271
Garden Warbler 17 265
Red-legged Partridge 17 259
Sparrowhawk 14 220
Treecreeper 14 214
Nuthatch 13 204
Lesser Whitethroat 13 200
Wheatear 12 187
Grey Partridge 12 183
Canada Goose 12 183
Oystercatcher 11 175
Spotted Flycatcher 11 172
Sedge Warbler 10 160
Corn Bunting 10 151
Turtle Dove 9 139
Yellow Wagtail 8 126
Coot 8 126
Raven 8 118
Tree Sparrow 7 112
Mute Swan 7 112
Grey Wagtail 7 112
Tree Pipit 7 108
Common Gull 7 104
Snipe 7 103
B. Between 50 and 100 squares occupied
Siskin 6 96
Redstart 6 95
Tufted Duck 6 94
Lesser Redpoll 6 90
Marsh Tit 6 88
Shelduck 5 85
Red Grouse 5 81
Golden Plover 5 81
Cormorant 5 80
Hooded Crow 5 72
Little Owl 4 66
Whinchat 4 65
Willow Tit 4 63
Wood Warbler 4 63
Great Black-backed Gull 4 63
Redshank 4 57
Tawny Owl 4 56
Reed Warbler 4 56
Sand Martin 3 54
Common Sandpiper 3 54
Stonechat 3 53
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Table 2. Habitat composition of 200 metre transect sections within the Breeding Bird Survey 1994.
These figures do not reflect the habitat composition of the UK as a whole as they are not
corrected for regional variation in sampling effort.
Major Habitat
Percentage of transect
sections
Number of transect
sections
Farmland 54.2 8 175
Human Sites 15.7 2 372
Woodland 12.1 1 821
Heathland & Bogs 8.0 1 208
Grassland 5.0 756
Scrubland 2.3 347
Water Bodies 1.7 260
Inland Rock 0.6 83
Coastal 0.4 53
Miscellaneous 0.1 11
TOTAL 100.0 15 086
ACKNOWLEDGEMENTS
We thank the many BTO volunteers, both fieldworkers and regional organisers who have
contributed to the monitoring programmes discussed in this paper. The CBC and PCP were
supported under a contract from the JNCC (on behalf of English Nature, Countryside Council for
Wales, Scottish Natural Heritage, and under a contract from the Department of the Environment
(Northern Ireland). The project to evaluate sampling strategies was funded by RSPB. The BBS is
jointly funded by the BTO, the JNCC and the RSPB. We are grateful to the members of the BTO's
Integrated Population Monitoring Working Group, Prof. Steve Buckland, Drs. Rhys Green,
Nicholas Aebischer, John Goss-Custard and Dorian Moss for helpful advice. We also thank Drs.
David Stroud, Ken Smith, David Gibbons, Jeremy Greenwood, Will Peach, and Humphrey Crick
for helpful discussion. John Marchant, Dawn Balmer, Andy Wilson, and the late Dr Steve Carter
provided assistance.
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