Unequal food distribution among great egret Ardea alba nestlings: parental choice or sibling aggression?

In broods of great egrets Ardea alba and other birds with siblicidal nestlings, the first -hatched brood members generally secure far more food than do their juniors. This feeding advantage could be caused by parental favoritism, or by seniors attacking and thereby dominating their juniors. We investigated these possibilities by comparing how fathers and mothers allocated food among their offspring when chicks were free to fight versus when they were physically separated by a Plexiglas barrier. When free to fight, dominant nestlings received significantly more food than did their subordinates. When nestlings were separated, mothers, but not fathers, delivered significantly more food per meal to the beta (second-ranked) chick than to other nestlings. This is the first experimental evidence of differential feeding by parents in a species with aggressive nestlings