1.4. Evolutionary response to decrease of grouse

The goshawk, like all raptor species, shows a remarkable reverse sexual size dimorphism. The female can be 1.5 times larger than the male. Typically, in goshawks and also in other raptors, the roles of the sexes are clearly segregated during the breeding season. The male provides food for the female and offspring for most of the breeding season (Newton 1979). Thus food data, which is collected at that time, reflects primarily the prey choice of males. Because of the large size difference between mates, divergence in the diets of sexes could be expected (Reynolds 1972). Data gathered during the non-breeding season sometimes showed no difference in prey choice between sexes (Opdam et al. 1977, Widen 1987) but implied differences in some cases (Kenward et al. 1981, IV). The avoidance of food competition between sexes has been a favoured explanation for the sexual size dimorphism in raptors (Reynolds 1972). Numerous hypotheses have been put forward to explain this unique reversed dimorphism in hawks and owls (Hakkarainen et al. 1991). However, many of them remain untestable or can be tested only indirectly in ecological time by studying the reproductive output of pairs that have different male/female size ratios (Hakkarainen et al. 1991). Because only the male delivers prey for the brood during the critical period of hatching, its hunting skills are of essential importance for the survival of nestlings. This time may also be a period of strong natural selection, affecting or maintaining the reversed nature of size dimorphism in hawks or even their morphology (III). The agility of the male, partly due to its small size, is considered to be important in this sense (Storer 1966, Reynolds 1972, Andersson & Norberg 1981, Hakkarainen et al. 1995). Due to the decline of grouse populations their proportion in goshawks diet has respectively decreased. Alternative prey types are smaller, on average, than grouse. Therefore small sized males could be more efficient capturing alternative prey, which would have resulted in smaller males at present than in the good grouse years of the 1960’s and 1970’s. Females, on the contrary, do not start hunting until late in the nestling phase. In winter they rely heavily on hares as a stable food, which has increased rather than decreased. Therefore less or no changes in female size are expected.