NUMBERS, BREEDING PERFORMANCE, AND DIET OF THE SPARROWHAWK ACCIPITER NISUS AND MERLIN FALCO-COLUMBARIUS IN RELATION TO CONE CROPS AND SEED-EATING FINCHES

Spruce Picea spp, trees in Kielder Forest produced a heavy cone crop i n 1990/1991 but not in 1991/1992. Common crossbills Loxia curvirostra and siskins Carduelis spinus rely upon conifer seed for food, and cons equently large numbers were present in the good cone year. The abundan ce of these two finches was one of the main reasons why songbird densi ty was four times higher in 1991 than in 1992. The numbers and breedin g productivity of two raptors that specialise in hunting songbirds wer e also monitored in both years, Sparrowhawks Accipiter nisus appeared to respond to this increased food supply. In 1991 they fed on crossbil ls and siskins, and increased their breeding numbers and productivity so that seven times more chicks were reared than in 1992. These result s indicate that sparrowhawk numbers are likely to fluctuate far more i n large spruce forests than in other habitats in Britain where food su pply is more stable. To ensure that spectacular ''crossbill years'' oc cur in future it is important to maintain a diversity of coniferous sp ecies in forests, including appreciable areas of Norway spruce Picea a bies, Merlins Falco columbarius did not switch to feeding on crossbill s and siskins, and their breeding numbers and performance were similar between years, Moorland birds were their main food, although consider able numbers of woodland songbirds were also taken in early spring. Th ese results suggest that forests provide merlins with some prey at a t ime of year when moorland birds are scarce. Thus, the planting of larg e conifer forests may not be detrimental to merlin populations, provid ed that extensive areas of moorland remain nearby.