The relative roles of hunger and size asymmetry in sibling aggression between nestling ospreys, Pandion haliaetus

Abstract

The food-amount hypothesis, proposed to account for sibling aggression in brood-reducing bird species, predicts that broods will be aggressive when hungry, and that as a result, senior siblings will be able to take a greater share of the total available food. We performed a field experiment to test these predictions in ospreys, Pandion haliaetus. Broods were removed from their natal nests and subjected to a series of experimental trials. Each trial consisted of a 3.5-h starvation period followed by a feeding or sham-feeding treatment designed to manipulate the hunger of the nestlings. Following treatment, broods were placed in a viewing nest along with a fish. While the viewing-nest mother fed the fish to the brood, we recorded food allocation and aggresssion. Each of 10 broods experienced a number (one to six) of feeding and sham-feeding treatments over a period of 2 days and was then returned to its natal nest. Broods were slightly but significantly more aggressive after sham-feeding treatments, and senior siblings took a greater share of the meal. The tremendous variation in the level of sibling aggression observed between broods was inversely related to brood mass asymmetry but not to condition, suggesting that siblings adjusted the rate of aggression according to the level of competition within their broods.