Bird Populations (Collins New Naturalist Library, Book 124) by Ian Newton
Author:Ian Newton
Language: eng
Format: epub
Publisher: HarperCollins Publishers
Published: 2013-08-19T00:00:00+00:00
Nest-sites and predators
To protect their nests from predators, some bird species rely on camouflage, others on dense cover, and yet others on relatively inaccessible sites, such as high trees or cliffs, which give security at least against some mammalian predators. However, species that normally use only secure sites sometimes use less safe sites in areas that lack mammalian predators. Consider for example Peregrines, not as predators but as prey. Throughout their range, Peregrines nest mainly on cliffs, which protect them from mammalian predators, including humans. But in areas where natural predators and people are few, Peregrines will accept lesser crags, earth slopes or even low mounds on flat ground or in bogs (Newton 1979). In recent years, Peregrines in Britain have occasionally nested on slopes or level ground, but the habit has not persisted, presumably because such nests usually fail through predation. It is seemingly the risk of predation that restricts the variety of places where Peregrines are prepared to nest, and thus limits their distribution and numbers.
The nest-site preferences of other raptors are also related to predation risks, with species accepting ground sites only where mammalian predators pose no threat (Newton 1979). The minimum requirements of security sometimes change with time as disturbance and predation pressures change. In some parts of the world, Ospreys nest on the ground on offshore islands lacking mammalian predators, but they disappeared from a treeless island off California following its colonisation by Coyotes (Newton 1979). Individuals in many populations occasionally attempt to nest in less safe sites but seldom succeed. But if they do succeed, the habit might then spread and lead to local expansion in distribution and density. For example, among Kestrels on Orkney, the first ground nest was recorded in 1945. The habit then spread rapidly and, once a site was established it remained in continuous occupation so that, by 1955, 19 such sites were known, all in tall heather, in cracks in banks or Rabbit holes (Balfour 1955). This enabled Kestrels to occupy areas not otherwise available, and thereby increase their numbers.
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