Abstract
Fratricide in eagles (Accipitridae: Accipitrinae) is either obligate [second hatched nestling (C2) always dies] or facultative (C2 occasionally dies) and appears to be a function of relative size differences between siblings. Several factors, including time between hatching, differences in hatching weights, and possibly the sex of the nestling in conjunction with its hatching sequence, influence size difference. Size differences are modified further by feeding rates of young by adults. These factors determine the relative differences in locomotor development and coordination and thus the ability of the first-hatched nestling to control access to and monopolize parent-provided resources. Significantly greater (P < 0.0001) within-clutch volume differences (an indicator of disparity in hatching weight) and greater time between hatching (mode of 3 versus 2 days) are found in eagles in which fratricide is obligate (three species) than in those in which fratricide is facultative (eight species). We suggest that these differences are proximate mechanisms that account for observed differences in the frequency of fratricide in eagles. Although size difference is determined in part by the sex and hatching sequence of the nestling, the role of offspring sex in fractricide is not clear.
