| Summary: | Seasonal changes in the diet of the Goshawk Accipiter gentilis during the breeding season were studied in northern Finland from 1988 to 1994. The Goshawk preyed mostly on grouse during the nest-building and the incubation period (60-70% by weight). Then shifting first to ducks and later during the nestling time to corvids, thrushes and pigeons. During fledging predation shifted to grouse chicks and leverets . Goshawks preferred grouse, mostly Willow Grouse Lagopus lagopus and Hazel Grouse Bonasa bonasia in the early phase of breeding. The preference for Black Grouse Tetrao tetrix, especially that of the females, increased throughout the breeding season . Capercaillies Tetrao urogallus on the contrary were avoided. Other prey species or groups had sharp `preference peaks' at some part of the breeding season, ducks in May, corvids in June (nestling phase), pigeons in July and grouse chicks in August. Preferences found were not well explained by the predictions of the Optimal Diet Theory (active decisions of the predator to attack), but rather by the changes in the behaviour and the vulnerability of the prey. Goshawks adjusted their breeding time, following Perrins' (1970) theory, to start breeding as early as possible, thus ensuring the maximal food availability for the post-fledged juveniles.
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