PREY SELECTION BY SPARROWHAWKS ACCIPITER-NISUS AND CHARACTERISTICS OFVULNERABLE PREY

Prey selection of the Sparrowhawk (Accipiter nisus) was studied in nor thern Finland during 1982-1993. A total of 540 prey items tall birds) were recorded from the surroundings of 12 nests. For each prey species a vulnerability index was calculated as the ratio between its proport ion in the Sparrowhawk's diet and its proportion in the land bird comm unity (data from the whole of the Oulanka National Park, and spruce fo rests only). The average prey individual weighed 52 g, whereas the ave rage breeding bird weighed 36 g in the spruce forests and 24 g in the whole of the Oulanka National Park area. Prey vulnerability increased with increasing prey body mass. The result could be interpreted in the light of the optimal foraging theory, large prey being the most profi table among the Sparrowhawks' prey-size range. In the whole of Oulanka data, open habitat species were caught relatively more frequently tha n forest species, and prey vulnerability correlated negatively with pr ey abundance. Foraging behaviour and the nest site of the prey species were not related to their vulnerability. Phylogenetic analyses reveal ed a significant correlation between prey vulnerability and plumage br ightness, after the effects of body mass and abundance of prey species were controlled for. Higher relative predation risk of bright species give support to the 'sexual selection' hypothesis but not to the 'unp rofitable prey' hypothesis explaining prey vulnerability.