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

Abstract

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.

An audio–visual nest monitoring system for the study and manipulation of siblicide in bearded vultures Gypaetus barbatus on the island of Crete (Greece)

“…Future essential improvements to the system would include receiving the signal through a satellite link and using a back-up wind turbine, thus making the system suitable for cloudy days or areas with less sunlight. Bearded vulture siblicide Brood reduction through siblicide has been documented in many groups of avian species, including herons, boobies, raptors and corvids (Meyburg 1974;Braun and Hunt 1983;Mock et al 1987;Simmons 1988;Anderson 1989;Bryant and Tatner 1990;Gargett 1990;Boal and Bacorn 1994;Reynolds 1996;Wiebe 1996;Watson 1997;Clifford and Anderson 2001;Ferrer 2001). The existence of the marginal (additional) chick could be explained by three incentives of overproduction: (1) the resource-tracking (nutrient), (2) the sib-facilitation (ice-box) and (3) the replacement (insurance-egg) hypotheses (Alexander 1974;Mock and Forbes 1995).…”

“…In our study, although the extra food offered to the pair did not seem to affect the life expectancy of the second chick, the aggressive interactions between nestlings dropped significantly when the first chick was satiated; this provided the second chick with the opportunity to receive food unmolested. Given that fatal sibling aggression can decline when food is abundant (Meyburg 1974;Mock et al 1987;Poole 1989;Estes et al 1999), food provisioning to the breeding pair during the hatching period is probably the only management tool to delay the reduction of the brood. Intervention with the aim of rescuing the second chick would seem to be applicable when it is 1-2 days old, preferably the day following hatching when it is in its best physical condition, judging by its retaliation activity.…”

Sibling competition in a brood-tending leech

“…For example, parents often extracted the tissue from the capillary tubes and presented the naked food to their young ones, something we have never observed with snails. Manipulations of prey size are difficult to achieve experimentally even with large birds in laboratory settings (Mock et al 1987), and our use of natural variation among P. antipodar um snails seems to offer substantial, if imperfect, insight into prey-size effects. We conclude that, although the competitive behaviours may differ, prey size can exacerbate intrabrood competition in similar ways in organisms such as birds and glossiphoniid leeches.…”

Spatial and temporal variation of habitat and prey utilization in the Great White EgretArdea alba albaat Lake Neusiedl, Austria

“…One explanation is that, at the beginning of the breeding season, more egrets forage far away from the colony. A more probable explanation for the rise in foraging individuals is the increasing food demand of the young during the breeding season (Mock et al 1987).…”