Corvid Density and Nest Predation in Relation to Forest Fragmentation: A Landscape Perspective

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Abstract
The abundance and distribution of corvids (Common Raven, Corvus corax; Hooded Crow, C. corone; Jackdaw, C. monedula; Black—billed Magpie, Pica pica; and European Jay, Garrulus glandarius) were studied across a gradient from a landscape dominated by agricultural land to a landscape dominated by forest in south—central Sweden. The total density and the predation rate on dummy nests increased as the proportion of agricultural land increased. This supports the suggestion that the density of corvids increases as forest becomes fragmented and intermixed with agricultural land, causing an increase in nest predation in small forest fragments. However, the different species responded differently to the proportion of forest in the landscape. The two forest—living corvids, the Jay and Raven, were absent from small forest fragments in a matrix of agricultural land and mainly preyed upon dummy nests inside the large forest fragments. Thus, both of these species were habitat specialists restricted to forests. Three corvids basically inhabit agricultural land, the Hooded Crow, Jackdaw, and Magpie. The Jackdaw and the Magpie showed a strong preference for agricultural land and they mainly preyed upon dummy nests in agricultural land. On the other hand, the density of Hooded Crows was higher in landscapes with a mixture of agricultural land and forest than in landscapes dominated by either agricultural land or forest, indicating a use of both agricultural land and forest. Further, Hooded Crows regularly preyed upon nests in both forest habitat and agricultural land; it is a habitat generalist. Therefore, the Hooded Crow was the most important species in the corvid family, causing increased predation pressure close to forest—farmland edges and in small forest fragments surrounded by agricultural land.

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