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

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

Distribution and size of capercaillie leks in relation to old forest fragmentation.

Distribution and size of 38 capercaillie Tetrao urogallus leks were related to amount and configuration of old forest patches in two south-east Norwegian coniferous forests. The smallest occupied patch was 48 ha containing a solitary displaying cock. All patches larger than 1 km2 contained leks. Number of cocks per lek increased with increasing patch size. Number of leks per patch increased in a step-wise manner with one lek added for each 2.5-3 km2 increase in patch size. In large patches there was one lek per 3-5 km2 old forest, and density of lekking cocks was 2-2.5 per km2. In small patches density of cocks varied considerably. Density of cocks was not related to patch isolation or patch shape. However, among leks surrounded by 50-60% old forest within a 1 km radius, number of cocks increased with increasing old forest fine-graininess. We argue that when old forests cover more than 50%, a fine-grained mosaic may support higher densities of lekking cocks than a coarse-grained mosaic. Conversely, when old forests cover less than 50%, a fine-grained mosaic is unfavourable, because each old forest patch becomes too small and isolated. Finally, we present a predictive model of how old forest fragmentation influences density of leks, number of cocks per lek, and total density of cocks.