Does brood reduction provide nestling survivors with a food bonus?

Abstract

Siblicide is common in many asynchronously hatching birds, including brown pelicans,Pelecanus occidentalisMost adaptive models of siblicide, and of brood reduction in general, tacitly assume that parental deliveries of food remain fixed, and therefore that supplies to seniors remain unchanged or increase after a junior’s death. If parents match deliveries to brood size, however, then seniors may get less food following brood reduction. To test the assumption that parental deliveries remain constant, and to determine how brood reduction affected seniors, food deliveries to control (three-chick) and experimentally reduced (two-chick) broods of brown pelicans were compared. Parents brought less food to reduced than to control broods. Similarly, a literature review revealed that avian parents generally delivered less food to smaller than to larger broods of experimentally altered sizes. In brown pelicans, when food deliveries decreased following brood reduction, second-hatched (‘B’) chicks received less food, but first-hatched (‘A’) chicks’ supplies remained unchanged. B-chicks may have gained more in control broods by appropriating the extra food that last-hatched (‘C’) chicks otherwise would have gained. If brood reduction decreases fighting, nestlings may gain more net energy even if they receive less food. Fighting did not decrease significantly in reduced broods, perhaps because fledging averaged one chick/brood, so A and B-chicks still competed intensely. Alternatively, small sample sizes may have prevented detection of differences in fighting. Seniors did not concentrate their attacks on C-chicks, perhaps because seniors may benefit by delaying the decreases in parental food deliveries that follow brood reduction.