The Role of Brood Size in Regulating Egret Sibling Aggression

In siblicidal birds, overt aggression is used by nestlings to resolve competition for food, and the nestlings of most species diminish or abate their combat when that competition is relaxed. Previous studies of colonial herons and egrets, however, have indicated that the proximate cue on which the aggression was assumed to be predicated, namely, current food levels, has no effect on sibling fight rates; that is, "hunger" does not provide such a switch-off cue. We tested whether aggression might be curtailed in cattle egrets by the obvious alternative, attention to overall demand. When one of three chicks was temporarily removed from experimental nests, the remaining sibs virtually ceased fighting; when it was returned, hostilities resumed. This effect was not influenced directly by the rank of the removed chick, or by the amount of food available to the others (indeed, parents corrected for the change in brood size by bringing less food). By terminating aggression when brood size drops from three to two, the remaining nestlings stand to save the costs of needless fighting when they have little to gain from continuing attacks. We propose that species probably do best by relying on current food amount for the truncation of sibling fighting whenever that cue allows an accurate assessment of pending competition, with brood size used mainly as an alternative or backup system. Supply-based mechanisms should be more flexible and reversible than those predicated on demand. In species such as egrets, the senior siblings can probably achieve a modicum of "reversibility" by exercising restraint during attacks. By using nonfatal intimidation of nest mates, seniors probably control the competition while forestalling the irreversible step of premature siblicide.