Post-nestling behaviour in goshawks, Accipiter gentilis: I. The causes of dispersal

Abstract. To investigate fledging behaviour and proximate causes of dispersal, 221 radio-tagged goshawks were studied in the post-nestling period during 1980-1987. Offspring left the nest tree from 39 days after hatching. Only 2% of observations were more than 300 m from the nest when hawks were 40-65 days old, while they finish growing their main flight feathers, compared with 26% of pre-dispersal records in the following 25 days. Dispersal from the nest area was abrupt, at 65-90 days old for 90% of hawks and by 95 days after hatching for 98%. Females dispersed a week later than males. Young hawks given supplementary food dispersed later than others in the same area, but not later than 95 days after hatching. All hawks that were fed artificially after the removal of their parents also left by this age. It is concluded that dispersal was enabled by completion of feather growth and was accelerated by food shortage, but probably resulted from behavioural maturation if food was abundant.