Flexibility in the development of heron sibling aggression: an intraspecific test of the prey-size hypothesis

Abstract

Sibling aggression in nestling Arceidae has been associated with a diet of monopolizably small prey items: in a Texas colony two egret species that eat small prey are siblicidal, while adjacent broods of great blue herons, Ardea herodias, eat very large prey and seldom fight. In a previous experiment these herons showed considerable developmental plasticity, acquiring egret-like behaviour (direct feeding and fatal sibling aggression) when foster-reared on a small-prey diet. The prey-size hypothesis was further tested by studying a Quebec colony of great blue herons that take small prey as their natural diet. As predicted, these nestlings fought much more than their Texas conspecifics (an increase of 800–900%) and learned to catch the discrete food boluses directly from the parent's bill. Thus, early-hatched/senior brood members maintained a competitive advantage over younger siblings, even after losing their initial handling speed advantage, and they consumed disproportionate shares of the food. It is proposed that sibling aggression in this species is adaptively facultative, developing only when food is sufficiently small to be economically defended via aggression.