Capercaillie breeding success in relation to forest habitat and predator abundance

1. The capercaillie Tetrao urogallus is widely valued as a game bird and an indicator of forest ecosystem quality. In Scotland, its numbers have declined since the 1970s, due primarily to poor breeding success. We investigated whether differences in breeding success among forests are related to predator abundance or to aspects of forest structure, with a view to identifying beneficial management practices. 2. Capercaillie breeding success was estimated in 7-14 forest areas in Scotland each year during 1991-2001. Forest habitat, including ground vegetation, and indices of predator abundance were assessed in each of the 14 areas in 1995. 3. Capercaillie breeding success increased with increasing bilberry Vaccinium myrtillus ground cover, but did not improve further above 15-20% cover. 4. The main predators were carrion crow Corvus corone, fox Vulpes vulpes and raptors. Indices of their abundance were confounded such that their respective effects on capercaillie breeding success could not readily be distinguished. However, capercaillie breeding success was negatively related to a principal component score that represented the combined abundance of crows, foxes and, to a lesser extent, raptors. 5. A management trial in one forest showed that capercaillie bred better when most crows and some foxes were killed than when they were not. 6. An index of pine marten abundance was related neither to the other predator indices nor to capercaillie breeding success. 7. Synthesis and applications. Capercaillie reared more young in forests with more bilberry and fewer predators. Management to improve the breeding success of capercaillie should aim for at least 15-20% bilberry ground cover, and legal control of crows and foxes.