Causes and consequences of breeding dispersal in the Sparrowhawk Accipiter nisus

Sparrowhawks Accipiter nisus were present on particular territories for only one breeding season, but others were present on the same territory for up to eight seasons. Short periods of occupancy were due partly to mortality and partly to about one third of surviving birds of both sexes changing territories between one breeding season and the next. The mean number of territories used per individual breeder in the study area increased from one territory in one-year birds to 2 .8 territories in birds present for seven or more years . A change of territory was usually associated with a change of mate. The tendency to stay on the same nesting territory, rather than move to a different territory, increased progressively with age (in females up to the oldest age groups of 7-10 years). Within any one age-group, birds were more likely to change nesting territories after a breeding failure than after a success . As a group, birds which stayed on the same territories from one year to the next showed high nest success, but no improvement between years, whereas females which changed territories showed improved success in the year after the move. While movements of breeding birds between nesting territories were related to the age and experience of the individual concerned, by inference such individual movements affected the long-term occupancy of territories, as we ll as the overall breeding success and distribution of the population. There was no evidence that breeding dispersal, in terms of the proportions of breeders that changed territories each year, was density dependent.

Newton I . 2001, Ardea 89(special issue) : 143-154