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Factors affecting the survival of
Birds of Conservation Concern
by David Leech |
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5.2.2 Agricultural
intensification |
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| Agricultural intensification during the post-war period has greatly
reduced the amount of food available to foraging birds, especially
granivorous species, on farmland habitats (O'Connor and Shrubb, 1986;
Donald, 1998; Robinson and Sutherland, 2002). |
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- Increased use of herbicides and insecticides has led to a decrease
in the abundance of weeds growing on arable land and therefore
to a decrease in both the availability of seeds to foraging birds
(Figure 5.1) and the abundance of invertebrate prey.
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Graph showing long-term
trend in seed density in arable soils.
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- Destruction of hedgerows and field margins has reduced the area
available for colonisation by seed-producing weed species and
invertebrates.
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- As the efficiency of harvesting has increased, the amount of
grain spillage has decreased ten-fold.
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- Planting of winter cereal crops has also increased, reducing
the amount of fallow land and particularly the area of seed-rich
stubble on which birds are able to forage during the winter. Stubble
has been identified as the preferred winter foraging habitat for
a variety of farmland bird species including corn buntings (Donald
and Evans, 1994) and reed buntings (Wilson et al., 1997b).
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| At present, little direct evidence exists to link variation
in the survival rates of declining farmland birds with specific changes
in agricultural practices (see Section 4). This is partly due to the
fact that data concerning such changes (eg variation in seed abundance)
are not available at a sufficient temporal and spatial resoluation
to permit reliable analyses of relationships with long-term variation
in survival rates. However, the results of several studies suggest
that certain land management practices may affect population sizes
of some species. |
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- The results of studies by Siriwardena et al. (1998, 1999)
suggest that declines in the population sizes of a variety of
farmland bird species are due to decreasing survival rates, which
in turn may be related to changes in their food supply.
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- Several farmland bird species have been observed to forage preferentially
on seed-rich fields (Donald and Evans, 1994; Wilson et al.,
1997; Donald and Vickery, 2000). A reduction in seed density may
result in reduced intake or increased foraging times, potentially
leading to a decrease in survival rates, particularly during adverse
weather conditions (Robinson, 2001).
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- Donald (1997) observed that the area of spring cereals sown
annually in the UK between 1968 and 1993, and therefore the area
covered by stubble during the winter, was positively correlated
with the abundance of corn buntings at a national scale (Figure
5.2). However, this study provides only weak evidence for a link
between seed availability and corn bunting survival as many other
factors with the potential to influence the size of the population
may also have displayed variation over this period, eg field sizes,
pesticide use, hedge removal.
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Graph showing the
positive relationship between area of spring sown cereals and
corn bunting abundance.
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A reduction in seed
availability has been proposed as the principle factor responsible
for the decline of the corn bunting population.
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- Evans (1997) reported an increase in the number of cirl bunting
pairs from 120 to 350 over a 4-year period in response to the
increased provision of field margins and winter stubbles.
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- Examining the population recovery, Peach et al. (2001)
found that cirl bunting abundance had increased by 83% in areas
where field margins and stubble had been provided, compared to
a population increase of just 2% in 'unimproved' areas.
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| Further investigation is needed to determine whether increased food
availability over the winter period affects population size by decreasing
mortality rates or by increasing brood productivity in these species.
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