Food, Growth, Survival and Sex Ratio of Nestling Hen Harriers Circus c. cyaneus in Orkney

Abstract

Young rabbits Oryctolagus cuniculus caught by the female hen harrier C. cyaneus were the main prey of nestlings on Orkney from 1975-1977. Males caught smaller prey items. As males provide most of the food when chicks are small (when mortality among chicks is highest) voles Microtus arvalis orcadensis may be an important item at this time unless suitable alternative prey are available. Growth curves for males and females showed that the sexes are clearly separable by weight. Nestling mortality was most closely associated with the order of hatching, with the last to hatch generally the first to die. There was an underlying trend for an imbalance in the sex ratio of nestlings in favor of females since 1953, which was most clearly seen in broods of 3 young. Differential mortality is suggested as the most likely explanation, although no evidence was found for this from a detailed analysis of a sample of 79 broods from 1975-1978 for which full details were known.