The role of parents in sibilicidal brood reduction of two booby species

Abstract

Parental regulation of the probability and timing of avian brood reduction is expected to exist and evolve because natural selection should favor parents that match brood size to food availability most efficiently. Since food availability varies among species, interspecific variation in this regulation is expected. Comparative study of species with different brood reduction systems is one way to test the hypothesis that these mechanisms evolve as adaptations. Previous work on obligately siblicidal Masked Boobies (Sula dactylatra) and facultatively siblicidal Blue-footed Boobies (S. nebouxii) has shown that their different hatching asynchronies cannot fully explain the qualitative difference in their brood reduction systems. In this study, I report interspecific differences in nest shape that appear to contribute to early siblicide in Masked Boobies, but that suppress early siblicide in Blue-footed Boobies. Blue- footed Booby nest shape is closely regulated by parents. Differences in egg sizes of nest mates do not appear to contribute to the difference in social systems.