Abstract

I studied the ecology of the goshawk-grouse relationship in Oulu, northern Finland, during and outside the breeding season, by radio-telemetry. This included museum samples of goshawk to obtain a better ecological as well as a better evolutionary understanding of it.

The proportion of grouse in the diet of goshawks has decreased since the 1960’s, in accordance with the decline of grouse populations. The main prey groups replacing the lacking grouse were corvids, squirrels and hares. The proportion of grouse was highest in spring and it decreased towards the end of the nestling phase. The most preferred grouse species were hazel grouse Bonasa bonasia and willow grouse Lagopus lagopus. Preferences for different prey types are not explained by active choices of goshawk, but by changes in the vulnerability of the prey species. The nestling phase, when food demand is highest, is not adjusted to when prey supply is highest, but before it.

The size and shape of the goshawks has changed from the 1960’s. Adult males became smaller but females larger. Both became relatively longer winged and tailed. Decrease of male’s size may be a response to the change in the food supply. Prey types replacing grouse are generally smaller, which may cause the change in the male’s morphology. Females being less active during the breeding season may not be affected. For the female to be larger is advantageous in winter when they kill ‘over large’ prey like mountain hares Lepus timidus and capercaillie cocks Tetrao urogallus.

Wintering goshawks were mainly females in adult plumage that tended to stay in the study area. However, only one third bred locally. More than one quarter of all hawks died during the study. Although known to be inhabitants of old forests, which this study supports, goshawks are fairly well adapted to mosaic landscape resulting from modern forestry, providing that suitable sized prey is available. Females have less problems, probably because hares, the main winter prey for females, are not affected negatively by forestry, like grouse and squirrels are, the main prey for males.

Goshawks have a remarkable impact on grouse populations, especially when non-territorial hawks, ‘floaters’ are also included. About one half of the total mortality rate of grouse may be due to goshawk predation. Goshawk predation accords to predictions of general predation theory and may be a noticeable factor contributing to cyclicity in grouse.