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

Area of Science:

• Behavioral Ecology
• Ornithology
• Evolutionary Biology

Background:

• The "Food Amount Hypothesis" (FAH) posits that sibling aggression in birds is inversely related to food availability, mediated by chick hunger.
• This hypothesis suggests food shortages influence the lethality of sibling aggression and sibling-induced mortality.
• Facultatively siblicidal species, where aggression is not always fatal, provide a model to test this hypothesis.

Purpose of the Study:

• To evaluate the effects of food amount on aggression and survival in facultatively siblicidal Ardeidae (herons and egrets).
• To test the proximate and ultimate predictions of the Food Amount Hypothesis in Great Egrets (Casmerodius albus) and Great Blue Herons (Ardea herodias).

Main Methods:

• Observed sibling aggression and brood success in unmanipulated Great Egret and Great Blue Heron broods.
• Experimentally increased food amount in wild egret broods and decreased food amount in captive egret broods.
• Assessed fighting rates, brood reduction, and overall brood success in response to food manipulations.

Main Results:

• Sibling aggression did not decrease with increased food; a weak positive relationship was observed.
• In egrets, youngest sibling's competitive vigor and fighting rate were key predictors of brood success.
• Increased food in wild egrets lowered brood reduction without altering fighting rates; decreased food in captive egrets increased brood reduction but not fighting rates.

Conclusions:

• Food amount has minimal direct impact on fighting behavior in these Ardeidae species, but significantly influences chick survival.
• The proximate effects of food amount on aggression should be separated from its ultimate effects on survival in these species.
• The FAH may apply to other facultatively siblicidal birds with slower food fluctuations, but not obligate siblicidal species.