Demography of an Increasing Population of Sparrowhawks

Abstract

SUMMARY (1) The recolonization of an area in east-central England by sparrowhawks Accipiter nisus (L.) was studied after the local population had been eliminated, almost certainly by organochlorine pesticides. Over a 10-year period, nest numbers increased rapidly to begin with and then more slowly, with a mean rate of 21% per year. (2) Over the whole 10 years, the estimated annual survival of breeders was at least 76% for males and 74% for females, and mean productivity was 2*1 young per nest. Throughout, the breeding population contained a large proportion of first-year birds, with an overall figure of 35% of all male breeders and 25% of all female breeders. As the breeding population grew, young raised in the area tended to disperse ever greater distances from their natal sites. (3) Comparison of the increasing population with stable and decreasing populations in southern Scotland showed that differences in population trend were associated mainly with differences in the recruitment of new breeders (greatest in the increasing and lowest in the decreasing population) and in age of first breeding (earliest in the increasing and latest in the decreasing population). There were also differences in dispersal distances (shorter in the increasing population), and in the annual survival of breeders (greater in the increasing population). In contrast, differences in breeding success between areas were slight, and non-significant.