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Factors affecting the survival of
Birds of Conservation Concern
by David Leech |
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6.1 Obtaining accurate estimates of survival rates
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| Good estimates of over-winter survival should be obtainable
from mark-recapture data for sedentary species, such as song thrush,
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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| The long-running BTO
National Ringing Scheme and the Constant
Effort Site (CES) scheme both help to determine survival rates
by recording the recapture of previously marked birds. However, the
Retrapping Adults for Survival (RAS) project implemented by the
BTO in 1998 aims to increase the amount of data collected on the survival
rates of bird populations by intensive, long-term monitoring of individual
species over small spatial scales at multiple sites across the UK.
At each site, a team of ringers attempts to catch all the adults of
a particular species during each year whilst maintaining a similar
level of catching effort, thus allowing assessment of mortality rates
and identification of surviving individuals between years. At present,
over 120 RAS studies have been registered covering over 40 species,
including 13 studies on 8 red-listed species and 39 studies on 16
amber-listed species. |
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31 October, 2007
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