Breeding success of black grouse and capercaillie in relation to mammalian predator densities on two spatial scales

We studied the spatial covariation of breeding success of black grouse and capercaillie and densities of mammalian generalist predators in Finland. Our study was based on six years' data from the Finnish wildlife triangle censuses (1989-1994). At the nation-wide level (100 x 100 km grids), the proportion of hens with a brood in August covaried regionally in the two species and decreased towards southern Finland, whereas the brood sizes were not related to latitude. The relative densities of red fox and pine marten correlated negatively with the proportion of grouse hens with a brood and were two- to three-fold higher in southernmost Finland than elsewhere in the country. The relative density of stoat, however, increased with latitude. The results support the hypothesis that higher generalist predator densities are responsible for lower breeding success of forest dwelling grouse moving southwards in Fennoscandia. The results also suggest that the stoat is of minor importance as nest and brood predator compared to red fox and pine marten. In northern Finland, on a smaller spatial scale (about 25 km(2)), the increasing red fox density decreased the probability of an observed grouse hen being with a brood in low vole years, whereas in years of high vole densities no effect was detected in logistic regression analysis. In contrast to northern results, the red fox density in southern Finland, although generally higher, did not predict the spatial variation in breeding success of grouse either in high or in low vole years. The brood size of grouse was not related to red fox density. Our results agree with the alternative prey hypothesis in northern, but not in southern Finland. In addition to higher amplitude of vole cycles in northern than in southern Finland, this inconsistency can probably be related to regional differences in both prey and predator communities.