Capacités prédatrices et budgets d’activité chez l’Aigle pêcheur Haliaeetus vocifer

Abstract

Ten pairs of Fish Eagles Haliaeetus vocifer taken among a dense population in Uganda, were followed at the end of the breeding season to compare the time and energy budgets of breeding and non breeding adults. Fish are usually obtained by diving from a waterside tree. If unsuccessful, the bird searches in flight, an energy-expensive but very rewarding method. The additional food necessary to feed the young is provided mainly by the male through fishing flights. Another source of increase in energy expenditure related to breeding activities is the more active territorial defense by adults with young than without young. The food consumption of the former is higher than that of the latter ; but the difference is smaller than the increase of the energetic cost of breeding (42 % and 64 % for 1 and 2 fledged young). Thus brood size seems to be limited by the ability of the parents to feed their young and maintain a territory of adequate size or quality, even in the optimal conditions studied. Immature birds suffer heavily from the adults’ territorial behaviour, which forces them to make extensive flights and confines them to marginal food sources. This may explain their high mortality rate (83 % during the first four years).