Sexual Size Dimorphism and Sibling Competition: Implications for Avian Sex Ratios

The blue-footed booby's (Sula nebouxii) first-hatched chick aggressively dominates and outgrows its sib and sometimes kills it when food is in short supply. Differential male mortality is expected since females grow to be 27% heavier than males and should have an advantage in sibling competition in mixed-sex broods. Surprisingly, when the male hatches first, even though his sister outgrows him, he usually sustains dominance over her throughout the nestling phase, and he grows and survives normally. When the sister hatches first, despite a large, persistent sibling size disparity the male grows and survives normally. These observations contradict the prevailing hypothesis that in sexually dimorphic birds that practice siblicidal brood reduction the smaller sex suffers differential mortality as the result of its disadvantage in sibling conflict in mixed-sex broods. Such species consequently are not expected to modify their primary sex ratio or associate sex with hatching order in response to biased mortality in brother-sister conflict.