Factors affecting the survival of
Birds of Conservation Concern
by David Leech
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5.1 Natural
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5.1.1
Predation |
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Sparrowhawk numbers
have increased as persecution and organochlorine pesticide use
have decreased.
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Magpies frequently
feed on the eggs and chicks of breeding birds.
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| A reduction in the use of organochlorine pesticides since the 1960s,
together with a general decrease in the level of persecution by egg
collectors, hunters and game-keepers, has led to an increase in the
abundance of avian predators and nest-predators, eg sparrowhawks
(Accipiter nisus) (Marchant et al. 1990) and magpies
(Pica pica) (Newton and Haas, 1984), prompting the suggestion
that increased predation is responsible for observed declines of species
of conservation concern. Simultaneous decreases in the amount of cover
available due to hedge removal and destruction of field margins may
further increase the probability of individuals being predated as
they may be more easily observed by predators (Donald and Vickery,
2000). |
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- There is some evidence that predation by raptors is a major
factor limiting the population sizes of some wader species, at
least during the winter, including the amber-listed redshank (Tringa
totanus) (Cresswell and Whitfield, 1993). The size of red
grouse (Lagopus lagopus scoticus) populations may also
be limited by avian predators (Thirgood et al., 2000).
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Predation by raptors
may be responsible for limiting red grouse numbers.
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- However, Gooch et al. (1991) found no relationship between
magpie density and songbird breeding success. Thomson et al.
(1998) reported that patterns of songbird decline were similar
both in areas where predators occur and in those from which they
were absent, suggesting that increased rates of predation are
unlikely to be responsible for the observed trends.
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- Numbers of other prey species regularly taken by sparrowhawks,
including robin
(Erithacus rubicula), chaffinch
(Fringilla coelebs) and great
tit (Parus major) (Newton, 1986), have not decreased
over the same period even though the predation pressure experienced
by some of these species appears to be great (Gosler et al.,
1995).
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Great tit abundance
has increased over the last 25 years, despite increasing numbers
of one of its major predators, the sparrowhawk.
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- Furthermore, evidence from predator removal experiments (Côté
and Sutherland, 1997) indicates that avian populations generally
do not increase in size if predators are removed. This suggests
that the decrease in the size of the population due to predation
is compensated for by a subsequent increase in the survival rate
of the remaining birds, possibly due to reduced competition for
resources.
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| This evidence suggests that an increased probability of predation
is unlikely to be responsible for post-war declines in the population
size of the majority of red-listed and amber-listed bird species. |
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