(1) Tetraonid censuses were carried out in 1975-77 on randomly selected compass transects. The hazel grouse was clearly the most numerous tetraonid species in the study area of 500 km2, accounting for 74% of all tetraonid records. During its population lows the hazel grouse mainly occurred in its optimal habitats. The habitat amplitude was positively correlated with the abundance of the species, and during the increasing phase suboptimal habitats were filled at a faster rate than optimal habitats. The habitat distribution pattern may act to stabilize predation. (2) At the beginning of the study period, in 1974; the number of breeding goshawk pairs was twenty-five. In 1976-77 the tetraonids had an exceptionally deep population low and in consequence the goshawk population crashed to the level of about ten breeding pairs. However, there were no marked changes in the mean brood size of the goshawk during the study period 1974-81. (3) The hazel grouse is the most important single prey species of the goshawk. The percentage of hazel grouse biomass in the diet during the breeding season varied in 1974-81 between 4 and 34%. The proportion of tetraonids in the diet decreases strongly in the course of the summer, especially when the young of corvids fledge. (4) The goshawk showed no clear numerical response to fluctuations in hazel grouse densities. There was a marked functional response, but the form of the response curve was atypically concave. We argue that the shape of the curve would actually be sigmoid (Holling's type 3), if our data had included higher densities of hazel grouse. We also suggest that, in general, the shape of the functional response curve depends on the prey density and on the prey utilization rate of the predator, and that most types of response curves may be derived from the S-shaped sigmoid curve. (5) A typical delayed density-dependent relationship existed between the predation rate of the goshawk and the abundance of the hazel grouse. The average rate of predation on the adult hazel grouse population in the breeding season was estimated to be 12%. If the predation pressure was constant, this would mean an annual rate of 36%, but predation is probably heaviest in the spring.
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