Role of Sibling Aggression in Food Distribution to Nestling Cattle Egrets (Bubulcus ibis)

Sibling aggression occurs in a wide variety of asynchronously hatching bird species. In some, fights among siblings lead inevitably to death, in which case the benefits of winning are clear. In species where sibling aggression is common but usually not fatal, the benefits gained by winning and the methods used to achieve them are less obvious. In a Texas colony of Cattle Egrets (Bubulcus ibis), sibling aggression was frequent but siblicide rare. Parents rarely interfered with fights. Last-hatched chicks lost more fights and received less food than their elder siblings. Fighting limited the losers' immediate access to food and contributed to the senior sib's ability to monopolize boluses. These results are consistent with the hypothesis that monopolizable food can act as both a proximate and ultimate cause of sibling aggression. The main effect of sibling aggression lay in depressing food supplies to last-hatched chicks. First- and second-hatched sibs accrued roughly equal feeding advantages.