Nest loss in capercaillie and black grouse in relation to the small rodent cycle in southeast Norway

Summary The relationship between nest loss in boreal forest grouse and the fluctuations in small rodents was studied at Varaldskogen in southeast Norway during 1979–1986, covering two complete rodent cycles. Nest loss in capercaillie (N=174) and black grouse (N=81) was calculated according to Mayfield (1975) based on nests from radio-equipped hens (N=77) and nests found by other methods (N=178). Small rodent density was measured by snap trapping during spring and autumn. Losses varied as predicted by the classical alternative prey hypothesis (Hagen 1952 and Lack 1954, as elaborated by Angelstam et al. 1984): high losses during rodent crash years (85.5% capercaillie, 51% black grouse), and smaller losses during peak years (54.5% capercaillie, 32.5% black grouse). Losses were inversely related to autumn abundance of rodents in capercaillie (P<0.05), but the correlation was not significant for black grouse (0.10<P<0.20). In capercaillie, the only species with an adequate sample for analysis, no relationship was detected between spring density of rodents and nest loss. Losses during the prepeak years were nearly as high as during crash years, a result inconsistent with the model. We conclude that the numerical response of predators to their cyclic main prey (i.e. small rodents) probably play a main role during the low phase and prepeak year, whereas the dietary shift is most important during the peak and crash year of the cycle.