Numbers, breeding performance, and diet of the sparrowhawk Accipiter nisus and merlin Falco columbarius in relation to cone crops and seed-eating finches

Abstract

Spruce Picea spp. trees in Kielder Forest produced a heavy cone crop in 1990/1991 but not in 1991/1992. Common crossbills Loxia curvirostra and siskins Carduelis spinus rely upon conifer seed for food, and consequently large numbers were present in the good cone year. The abundance of these two finches was one of the main reasons why songbird density was four times higher in 1991 than in 1992. The numbers and breeding productivity of two raptors that specialise in hunting songbirds were also monitored in both years. Sparrowhawks Accipiter nisus appeared to respond to this increased food supply. In 1991 they fed on crossbills and siskins, and increased their breeding numbers and productivity so that seven times more chicks were reared than in 1992. These results indicate that sparrowhawk numbers are likely to fluctuate far more in large spruce forests than in other habitats in Britain where food supply is more stable. To ensure that spectacular “crossbill years” occur in future it is important to maintain a diversity of coniferous species in forests, including appreciable areas of Norway spruce Picea abies. Merlins Falco columbarius did not switch to feeding on crossbills and siskins, and their breeding numbers and performance were similar between years. Moorland birds were their main food, although considerable numbers of woodland songbirds were also taken in early spring. These results suggest that forests provide merlins with some prey at a time of year when moorland birds are scarce. Thus, the planting of large conifer forests may not be detrimental to merlin populations, provided that extensive areas of moorland remain nearby.