3.1. Dietary response of goshawks to decrease of grouse

In spite of a remarkable decrease in grouse since the 1960s (Linden & Rajala 1981), their proportion in the diet of northern goshawk has remained relatively high (I, II, V). In northern boreal forests grouse constitute the only sufficiently large and abundant prey for goshawk sized raptors. Really, the black grouse is the most important bird species in boreal forests if estimated by biomass (Järvinen et al. 1977). In natural conditions therefore, the goshawk has relatively little scope for switching to other prey if the main prey decreases (c.f. McCowan 1975, Rohner 1995). Although considered as a generalist predator (Marti et al. 1993), the goshawk is actually fairly specialised in the north. Particularly in winter time when migratory birds are absent (McCowan 1975, IV). The only possible alternatives are mountain hares and red squirrels (I,IV). Of these, mountain hare is too large a prey for male goshawks but for females they constituted the main prey in winter (IV). Therefore, one might expect that the goshawk would decrease in proportion with the grouse. This has probably not taken place, despite the fact the goshawk was also heavily persecuted until 1989, before it became totally protected (Haukioja & Haukioja 1970, Saurola 1976, 1985, Väisänen et al. 1996). How did the goshawk persist with such expansive harvesting and the simultaneous decrease of the main prey, grouse? Harvesting was probably not very harmful because it removed mainly juveniles. It was probably compensated for by improved survival and reproduction of breeding birds (Haukioja & Haukioja 1970, Kenward et al. 1991). Since protection the juvenile mortality rate has not decreased because the majority of them, in particular males, die of starvation (Tornberg & Virtanen 1997, IV). However, the continual decrease of grouse has caused problems for goshawks. In southern Finland their population density has decreased (Linden & Wikman 1983, Forsman & Ehrnsten 1985). Also, elsewhere in Fennoscandia goshawks have been reported to be declining markedly, especially in Norway (Tommeraas 1993, Halley 1996, Widen 1997). In the province of Oulu the breeding success of goshawks dipped in the 1980’s when grouse had an exceptionally long low phase, but it recovered in the late 1980’s. In my study area the number of occupied territories continually decreased during the study period in the 1990’s, but this might fit within the limits of normal population fluctuation (c.f. Haapala et al. 1994, I, V). Goshawks have partly been able to compensate for the loss of grouse by switching to other prey found near settlements (I, II). Corvids could, as a relatively large prey, be a realistic alternate prey in winter. They were, however, not found in the diet during the winter study, but brown rats that were taken from the city dump were found (IV). Probably corvids, mainly hooded crows, forming huge flocks in winter are not an easy prey for goshawks. The significance of corvids probably increases the further south you go. In southern Ostrobotnia in western Finland corvids were the main prey in nestling phase, constituting up to 40% of prey specimens (Tornberg, unpubl.). In Häme grouse constituted 50% in the spring diet in the 1950’s. Now that figure is about 15%. Corvids increased simultaneously in the spring diet from 10% to 28% and thrushes from 4% to 16% (Sulkava, pers. comm.). They were clearly the most important compensating prey type. In summer the changes in diet were not so pronounced.