Waarom kiest de Havik Accipiter gentilis in Białowieża (Oost-Polen) voor naaldbos?

In 50 km² of primaeval forest in East-Poland three pairs of Goshawk were located, breeding 2.6 and 3.6 km apart. The density of pairs was 4-5 times lower than in my Dutch study area, probably reflecting a poorer food base in Bialowieza. Nests were exclusively situated in the coniferous stands in the northern and eastern part of the study area, and were built in Scots pine Pinus sylvestris (2) and birch Betula sp. (1), fledging respectively 1, 2 and 3 young. Measurements on 4 chicks showed that they were in excellent condition, being on average 111% heavier than Dutch goshawks (corrected for age and sex). The condition of the young indicates that food-supply in the breeding season is not poorer than in The Netherlands. The low breeding density therefore probably relates to food-supply in winter, when many important prey species have migrated and the prey base has become very small. Adults carrying prey were mainly observed ascending from deciduous stands (5 out of 7), indicating a preference for hunting in deciduous forest tracts. The diet during the breeding season was also dominated by species that occur more abundantly in deciduous than in coniferous stands (Table 1). Common Buzzards largely had the same diet as the Goshawks (65% birds, of which 50% young thrushes; van Manen 2003), at least in 2003, Due to scarcity of voles and mice in 2003, Buzzards bred successfully only in deciduous stands and largely avoided coniferous stands (unlike Goshawks, which nested exclusively in coniferous stands). This difference in nest site choice is probably related to different migration strategies: Buzzards are migrants, leaving the area in winter, whereas Goshawks are residents. In winter, deciduous forests are largely devoid of profitable prey species. Hazelgrouse constitute the main winter prey of Goshawk in the Belarussian part of the Bialowieza forest & 1998), a species that is confined to coniferous forest. Habitat choice of Goshawks may therefore be largely determined by the distribution of this species, explaining the Goshawk’s preference for large expanses of coniferous forest. Buzzards, on the other hand, can select the richer deciduous stands in spring and summer, only penetrating and breeding in coniferous stands when voles and mice abound.