Siblicidal aggression and resource monopolization in birds.

In Texas, great egret Casmerodius albus chicks attack younger nestmates, often fatally (siblicide). By contrast, the young of neighboring great blue herons Ardea herodias seldom strike or kill siblings. These interspecific differences seem related to prey size: only fish provided by egret parents are small enough for chicks to monopolize (a process facilitated by aggression). Experimentally cross-fostered heron chicks raised on small prey by egret parents became siblicidal, but the reverse procedure of cross-fostering egret chicks did not reduce aggression or siblicide.