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Factors affecting the survival of
Birds of Conservation Concern
by David Leech
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6.2 Future
areas of research |
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| Over-winter survival has already been identified as
a potential key demographic rate driving the population declines of
several passerine species. However, little is known about the influence
of individual physiological condition on inter-annual variation in
over-winter survival rates and the relationship between these factors
requires further investigation. The relationship between body condition
and both survival and dispersal rates can be investigated using data
obtained from mark-recapture data for sedentary species and potentially
from a combination of recaptures and recoveries of dead birds for
more mobile species (Evans et al., 1999). However, such studies will
need to be carefully designed as estimates of survival may be confounded
by other factors, such as the dispersal of individuals after the breeding
season and variation in breeding site fidelity. |
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| Investigation of seasonal patterns of mortality may
also help to determine the potential causes of changes in survival
rates. For example, there is some evidence that survival rates of
seed-eating passerines may be particularly low in the late winter
period, when their food supply is at its most scarce (Crick et
al, 1991 BTO RR 80). Whilst comparisons of survival rates during
the winter and summer periods have been performed for some British
species, eg nuthatches (Sitta eurpoaea) (Nilsson, 1987), the
data has usually been collected intensively on relatively small populations.
Future studies should aim to calculate seasonal differences in survival
rates at larger spatial scales. Analysis of variation in the demographic
rates of populations between areas of different agricultural regimes
may also help to highlight the specific factors responsible for spatial
variation in survival rates of farmland species. |
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| Food abundance is likely to have a profound effect on
spatial variation in survival rates. Theory predicts that where patches
differ in food availability, individuals should distribute themselves
across those patches in a manner known as the Ideal Free Distribution,
where each individual maximises their feeding rate. From this it is
possible to determine the number of feeding birds on each, and how
much food they all eat. Consequently, it is possible to calculate
survival rates and the size of the population that an area is potentially
able to support. This approach has been used with a great deal of
success in modelling populations of geese (Vickery et al.,
1994) and waders (Gill et al., 2002), amongst others. Such
models can be applied to farmland birds and the BTO, under a DEFRA
contract, is currently investigating the importance of food supply
for seed-eating species in East Anglia. |
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