BREEDING SUCCESS OF BLACK GROUSE AND CAPERCAILLIE IN RELATION TO MAMMALIAN PREDATOR DENSITIES ON 2 SPATIAL SCALES

We studied the spatial covariation of breeding success of black grouse and capercaillie and densities of mammalian generalist predators in F inland. Our study was based on six years' data from the Finnish wildli fe triangle censuses (1989-1994). At the nation-wide level (100 x 100 km grids), the proportion of hens with a brood in August covaried regi onally in the two species and decreased towards southern Finland, wher eas the brood sizes were not related to latitude. The relative densiti es of red fox and pine marten correlated negatively with the proportio n of grouse hens with a brood and were two- to three-fold higher in so uthernmost Finland than elsewhere in the country. The relative density of stoat, however, increased with latitude. The results support the h ypothesis that higher generalist predator densities are responsible fo r lower breeding success of forest dwelling grouse moving southwards i n Fennoscandia. The results also suggest that the stoat is of minor im portance as nest and brood predator compared to red fox and pine marte n. In northern Finland, on a smaller spatial scale (about 25 km(2)), t he 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 anal ysis. In contrast to northern results, the red fox density in southern Finland, although generally higher, did not predict the spatial varia tion 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 res ults agree with the alternative prey hypothesis in northern, but not i n 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.