Broedende Zwarte Kraaien Corvus corone op heidevelden in Noord-Brabant en Friesland in 2004
Drentse vogels , Volume 18 - Issue 1 p. 38- 43
Two populations of Carrion Crow were studied in resp. the northern (Aekingerzond, 585 ha) and the southern Netherlands (Sint Anthonis, 239 ha). Both areas consist of dry heaths with scattered trees and are surrounded by coniferous plantations. Breeding density of Crows was five times higher in Sint Anthonis (Table 1). In both areas, nest sites were in full supply, but breeding density of predators was higher in Sint Anthonis than at Aekingerzond (Goshawk Accipiter gentilis 0.67 vs. 0.22/ 100 ha and Buzzard Buteo buteo 1.35 vs. 1.09/100 ha). At Aekingerzond, pairs started laying earlier and produced larger clutches and larger broods. However, the relation between laying date and number of fledglings in both areas suggests that at least to some extent both populations are subject to the same population dynamics (Fig.3). The difference in density in both areas can be explained by the models in Figure 4. Food supply prior to laying and during the nestling period at Aekingerzond probably equalled the situation in Sint Anthonis, as surmised from the early laying, large clutches and ditto broods and similar conditions of nestlings. For this reason the lower density at Aekingerzand is probably best explained by a higher mortality rate. This is counterintuitive with the higher density of predators in Sint Anthonis. However, density of predators per se may not be the crucial factor. Only when raptors face serious food shortage, as recorded in some areas in The Netherlands (among which Aekingerzand, but not Sint Anthonis), will depredation of Crows increase to such an extent that mortality caused by predation is additive to existing mortality rates. The lower density of predators at Aekingerzand coincides with food shortage for avian predators, hence resulting in higher predation rates than in Sint Anthonis where food supply is still sufficient to feed a larger predatory population.
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