Proximate and Ultimate Roles of Food Amount in Regulating Egret Sibling Aggression

In facultatively siblicidal bird species, the amount of food delivered by parent birds to their young ("food amount") has been assumed to be both an important proximate and ultimate cause of fatal aggression. The proximate "Food Amount Hypothesis" (FAH) contends that sibling aggression will vary inversely with the quantity of food delivered by the parents, presumably mediated by chick hunger. At the ultimate level, food shortages are expected to influence whether the combined effects of aggression and food control by older siblings will be fatal to the youngest brood member(s). We evaluated the effects of food amount on aggression and survival in facultatively siblicidal Ardeidae: (1) Sibling aggression in unmanipulated broods of Great Egrets (Casmerodius albus) and Great Blue Herons (Ardea herodias) did not decline with increasing amounts of food, as had been predicted; indeed, this relationship may even be weakly positive. (2) In egret broods, the strongest predictors of total brood success were competitive vigor of the youngest sib and fighting rate. (3) Wild egret broods whose food amount was experimentally doubled did not reduce fighting rates, but experienced significantly lower brood reduction than control broods. (4) Captive egret broods whose food amount was experimentally decreased did not increase fighting rates, but suffered significantly more brood reduction than control broods. (5) In the wild, food amount appears not to be a sufficiently stable parameter on which to base prudent sib—aggression strategies. We conclude that food amount has little direct influence on fighting behavior in these birds, though it consistently influences chick survival. Thus the proximate effects of this ecological variable must be divorced from its ultimate role, at least in species such as these ardeids and in obligate—siblicidal species. However, the FAH may well account for sib—fighting variation in other facultatively siblicidal birds, such as Blue—footed Boobies (Sula nebouxii), Black—legged Kittiwakes (Rissa tridactyla), Ospreys (Pandion haliaetus), and South Polar Skuas (Catharacta maccormicki). It is predicted that sibling aggression will be food—amount dependent in species whose food fluctuations are relatively slow. An attempt to manipulate prey monopolizability (bolus size) in captive Great Egrets was unsuccessful, either because sib aggression in the laboratory was so inflated as to swamp prey—size effects or because this species simply lacks sufficient developmental flexibility in its intrabrood fighting behavior. If the latter were true, it would reveal a striking difference between Great Egrets and Great Blue Herons, which have shown considerable (adaptive) plasticity in this regard.