3.3. Evolutionary response of goshawks to decrease of grouse

The capability to catch small avian prey depends a great deal on the agility of the raptor. Being smaller is then beneficial (Andersson & Norberg 1981). One of the earliest hypothesis for reversed sexual size-dimorphism in raptors was the so-called small male hypothesis, the reasoning is based on the males higher capability to catch small prey, which are encountered most frequently (Storer 1966, Reynolds 1972, Andersson & Norberg 1981, von Schantz & Nilsson 1981, Ydenberg & Forbes 1991, Hakkarainen et al. 1991). As already stated this skill may be crucial in early nestling phase. Because large grouse have remarkably decreased, goshawks have switched to corvids and passerines, which are smaller and have different flying properties than grouse (I,II). They are probably more capable of sudden turns than grouse, which rely more on power flight; rapid spurts and out-climbing (Pennycuick et al. 1995). Thus small, agile hawks could be better adapted to present conditions. This, really, seems to have happened in goshawks. Adult males have become smaller since the beginning of the 1960s (III). The faith of juveniles in their first year gives hints what is selected for and what against. All dead hawks, in particular ones that died of starvation, have failed in their life. Thus their quality might explain something about the properties that were selected against. Hawks that died in accidents may give a more random sample of the population. Probably goshawks have not yet been able to adapt to windows, cars and power lines. Starved juvenile hawks were larger than hawks that died in accidents in the 1960’s to 1970’s while the reverse was true in the 1980’s to 1990’s, which supports the idea of selection against large size of males at present. Not only did the size change but the shape, too. Adult males became longer winged and tailed, which also might increase agility and the ability to turn suddenly (Andersson & Norberg 1981). In contrast, the size of adults females increased. Females stay most of the breeding season in the nest, guarding the chicks. Inverse size relations in the sex of raptors is also assumed to depend on the female’s properties. Large females can store more fat than males and can stand periodic interruptions in prey deliveries during the incubation period (Lunberg 1986, Korpimäki 1986). Large females can also defend chicks against predators or even from aggressiveness from the male (Smith 1982, Mueller & Meier 1985). These aspects may well be a reason why the size of the female increased. However, it is known that a female’s size varies greatly over the distribution area, e.g. in Europe. Nest predators are the same in Europe, which goes against the latter theory. However, the predictability of prey becomes poorer in the north, which could, indeed, favour larger females according to the ‘starvation’ hypothesis (Lundberg 1986). Northern variants of the goshawk are larger than southern ones (Fischer 1980, Eck 1982). Female size could also be determined by the size of winter prey. Mountain hare was the most important prey for females in winter (IV). Also, capercaillie cocks are within a female’s reach. Because of the decrease of grouse, hares may have become a more important food source for females (Höglund 1964, Widen 1987, Kenward et al. 1981, IV). In North America, the snow-shoe hare is the main prey of goshawks. The snow-shoe hare weighs only about 1.5 kg so males can also catch it. The reason why the degree of sexual dimorphism in North American goshawks is less than in Europe may be caused by this (Storer 1966, Kenward 1996). The different diets of the sexes decreases the competition between them, which was also presented as a cause for RSD (Reynolds 1972, Newton 1979, Andersson & Norberg 1981). The problem with this hypothesis is, however, that avoiding competition will not explain the reversed nature of sexual size-dimorphism.