Bird Ringing for Science and Conservation - The European Union ...

Bird Ringing for Science and Conservation
Bird Ringing for Science and Conservation
Dispersal and population persistence
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Most European bird populations live in
habitats that are highly fragmented as a
result of human activities. The dynamics
and genetic diversity of populations inhabiting
such landscapes are often critically dependent
on dispersal patterns, as well as
on reproduction and survival within habitat
patches. To gain a better understanding
of how to manage these landscapes for
birds we need information on dispersal derived
from bird-ringing.
Two main types of dispersal are recognized
in population ecology. Natal dispersal
refers to movements between the place
of birth and that of first breeding, while
breeding dispersal refers to movements
between subsequent breeding attempts.
There are two complementary ways of
studying dispersal using bird ringing. Markrecapture
and mark-resight ing data can be
used to measure dispersal within local populations,
or between populations occupying
a limited number of colonies or habitat
patches. These studies provide a high resolution
picture of local movements but may
miss long-distance ones. In contrast, analyses
of ring recoveries provide a broad overview
of dispersal patterns including longdistance
movements, but may lack fine
detail. In both of these methods it is important
to control for variation in recording
effort.
Current knowledge of natal and breeding
dispersal is based on analyses of ringing
data. For most species natal dispersal is
greater than breeding dispersal, and species
with higher natal dispersal also tend to
move further between subsequent breeding
years. Average (geometric mean) dispersal
distances vary greatly between species.
For example, in Britain and Ireland
Blackcaps have an average natal dispersal
distance of 17,5 km while House Sparrows
move an average of only 0,2 km between
their natal and breeding sites. Dispersal
patterns are influenced mainly by the ecological
characteristics of individual species,
with those occupying more restricted and
patchy habitats showing greater dispersal.
Scarcer species generally occupy more restricted
and patchy habitats and this results
in a negative relationship between dispersal
and abundance. For similar reasons, dispersal
is greater amongst birds occupying
wetland habitats. Dispersal is also greater
in migrants than in residents, presumably
because of the opportunities for the
former to explore new areas. There is much
scope to explore such patterns further using
data from the EURING databank.
Natal dispersal
distances of Song
Thrushes measured
using ring recoveries
from Britain
and Ireland.
There is now increasing evidence from
fieldwork and mathematical modelling
that frequencies of occurrence and densities
of many bird species are lower within
habitat fragments than in large areas of
continuous habitat. For example, a study
in Northern Belgium found that Nuthatch
densities in forest fragments were about
half those in continuous areas of forest. In
this species, dispersal distances are larger
and territory vacancies are filled more
slowly in fragments than in continuous
habitat. Furthermore, areas where most
of the habitat is fragmented act as sinks,
with populations only being maintained by
immigration from more continuous habitat.
In order to manage populations within
fragmented landscapes it is vital to understand
these relationships between population
density, habitat quality and dispersal.
Understanding dispersal is equally important
for the conservation of colonial species
such as seabirds, where immigration
and emigration are key determinants of
colony size.
Dispersal also has important implications
for the maintenance of genetic diversity
within populations, and for rates of evolution
in changing environments. In most
bird species the greater natal dispersal of
females compared to males helps to reduce
inbreeding depression. A study of colourmarked
Great Reed Warblers in Sweden
found that low genetic variation and the
occurrence of inbreeding depression were
associated with restricted dispersal and
with a lack of any dispersal difference between
males and females. These genetic
studies further emphasise the importance
of improving our understanding of dispersal,
which remains poor relative to that of
other demographic processes. Large-scale
studies of marked birds should form an important
part of this research effort.
Alain Saunier
The relationship between average natal
dispersal distance and population size
for 75 species. More abundant species
generally occupy a wider range of habitats
and need to move less far in order
to find potential nesting sites.
The Nuthatch is a good example of a
species where patch occupancy in fragmented
woodland habitats is influenced
by dispersal.
after Paradis et al. 1998
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