BROWN PELICAN SIBLICIDE AND THE PREY-SIZE HYPOTHESIS

We asked whether the brown pelican (Pelecanus occidentalis) practices facultative brood reduction and tested two predictions of Mock's (1985) prey-size hypothesis: (1) if chicks take food directly from the parental mouth, nestmates should compete aggressively; (2) aggression between nestmates should increase during the developmental transition from indirect feeding (parents deposit food on the substrate) to direct feeding (parents pass food from mouth to mouth). Eggs in two-egg and three-egg clutches were laid and hatched 2 days apart. Junior (second- and third-hatched) chicks, fed less than their nestmates, grew more slowly and died more frequently. Of non-hatchling victims 74% were underweight, 55% bore peck wounds and 43% were found outside the nest, implying starvation, aggression and expulsion from the nest. Aggression occurred in all observed sibships, and junior chicks were subordinate in eight of ten cases. A progressive transition from indirect to direct feeding occurred during the 30 days after hatching, but there was no evidence of an increase in pecking during the transition. In conclusion, the brown pelican practices facultative brood reduction, and chick loss is mediated by sibling aggression as predicted by the prey-size hypothesis. Pecking during indirect feeds seems to contradict the prey-size hypothesis, but may serve partly to establish a relationship of dominance-subordinance.