Groei en ontwikkeling van jonge Sperwers Accipiter nisus: individuele variatie in broedstrategie
Drentse vogels , Volume 6 - Issue 1 p. 27- 36
Two nests of Sparrowhawks were closely monitored in 1992, especially during the nestling and fledgling stage. Wing length (maximum, straigthened and flattened), tail length, body mass (to the nearest gram), head+bill length, presence of the parents at the nest during controls, presence of egg tooth, condition of the young (crop) and several behavioural characteristics were monitored during daily late afternoon and evening visits (Appendix 1 and 2, Table 1, Fig. 1). The female of one pair started foraging halfway the nestling stage, the other female defended the nest aggressively (110 direct hits during attaques) and started only hunting after the young had fledged (Table 2). The young of both pairs had similar body masses at fledging, despite the positive effect of female hunting on mean prey weight brought to the nest (Table 3). Female Sparrowhawks can not deter Goshawks from robbing the nest, probably not even when breeding in dense stands or when being aggressive against potential predators. In 1991 and 1993, three of the four Sparrowhawk nests on Berkenheuvel were depredated by Goshawks. Predation is "encouraged” by recent habitat changes, in which exotic tree species are removed in order to create more natural forests. Stands with exotic trees, like Douglas fir and Norway spruce, are the preferred nesting sites of Sparrowhawks. Natural tree species, like Scots pine and oak, do not provide sufficient cover for Sparrowhawks in a Goshawk-dominated environment.
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