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Factors affecting the survival of
Birds of Conservation Concern
by David Leech |
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| 4.1 Breeding performance
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| The breeding performance of the population is dependent on: |
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- The proportion of the population that attempts to breed
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- The number of breeding attempts made per individual.
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- The productivity, in terms of number of offspring produced,
of each breeding attempt
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| There is strong evidence to suggest that decreased breeding success
is responsible for the long-term decline in numbers of some farmland
species of conservation concern. |
- The national decrease in the abundance of linnets
can be explained by a decrease in the reproductive success of
the population, which may have been caused by an increase in nest
failure at the egg stage (Crick et al., 1995; Siriwardena
et al., 2000).
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Linnet numbers may
have decreased due to an increased incidence of nest failure.
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- An increase in chick starvation caused by a reduction in the
abundance of prey items, notably sawfly larvae, has led to a decline
in grey partridge
numbers (Potts, 1986).
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- Increased chick mortality during silage harvesting is thought
to have negatively influenced the size of the UK corncrake population
(Green, 1999a).
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- Reduced fledging success has been proposed as the cause of a
decrease in lapwing
abundance since the 1960s, despite an increase in adult survival
rates over the same period (Peach et al., 1994)
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Decreased fledgling
survival may have resulted in the decline in lapwing numbers on
British upland farms.
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- Numbers of skylarks
fell by 51% between 1968 and 1995, a decline that may be due,
at least in part, to a reduction in the mean number of breeding
attempts made by individuals per season (Wilson et al.,
1997a; Chamberlain et al., 1999).
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A decrease in the
mean number of successful breeding attempts made by each individual
per season has contributed to the decline of the skylark.
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| However, several studies have identified a general tendency towards
an increase in the productivity of individual breeding attempts, in
terms of the number of fledglings produced, during periods of population
decline across a number of species of conservation concern, including
reed buntings, tree sparrows, turtle doves and corn buntings (Peach
et al., 1999; Siriwardena et al., 2000). The most likely
explanation for this increase in productivity is that infra-specific
competition during the nesting season is reduced as the size of the
population decreases. Alternatively, increased productivity may result
from a trend towards earlier laying dates, triggered by climatic changes,
which has been observed across a variety of British bird species over
the last 25 years (Crick et al., 1997; Crick and Sparks, 1999)
and is, possibly associated with the timing of reproduction of predator
species. |
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| Other factors, in particular survival rates, may therefore be important
in explaining variation in population sizes in these species. |
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