Siblicidal Brood Reduction: The Prey-Size Hypothesis

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Abstract
Siblicidal brood reduction involves sacrifice of very close kin (indirect fitness) for improved self-survival (direct fitness). In a comparison of 2 spp. [Casmerodius alkus and Ardea herodias] of herons (Ardeidae) in a Texas [USA] colony, great egret nestlings fought 18 times more often than adjacent great blue herons during the 1st month. Egret nestlings frequently killed the youngest of 3 or 4 siblings; herons seldom did. (The frequency estimates for sublicide in this egret population range from 18.8% [32/145] in a census sample to 52.9% [9/17] in nests subjected to close behavioral observation.) These interspecific differences in siblicidal aggression seem related to the degree to which the limiting resource, food, is potentially monopolizable by the nestlings. Ardeid parents eat mostly fish, which they transport to the nest internally (in crop and esophagus) and regurgitate for the young. The diet of Texas great egrets consisted almost entirely of very small individual fish that were delivered to the chicks in discrete boluses. By the age of .apprx. 2 wk, elder chicks could intercept most boluses and thus monopolize the food. This monopolization was greatly enhanced by fighting and intimidating younger siblings. Great blue heron prey tended to be too large for the young to intercept, which made effective monopolization much more difficult. A general hypothesis is advanced, linking adaptive siblicidal aggression to delivered food size (specifically, monopolizability). Some of its predictions have withstood experimental testing (Mock 1984b) and it accounts well for the known cases of siblicide in ardeids and other avian taxa.