Clutch size variation in the Nazca booby: a test of the egg quality hypothesis

In obligately siblicidal bird species, aggressive behavior by a dominant chick results in a fixed broad size of one, yet these species usually show clutch size variation between individuals. Simmons proposed that variation in clutch size in obligately siblicidal species is related to a trade-off between egg quality and egg quantity: some individuals produce a single highly hatchable egg, while others produce two small, lower quality eggs. We tested the egg quality hypothesis as an explanation for observed clutch size variation in the Nazca booby (Sula grand), an obligately siblicidal seabird. We tested the assumption that egg volume is positively correlated with hatchability and the prediction that eggs from one-egg clutches are larger than eggs from two-egg clutches. We did not find a positive relationship between egg volume and hatchability in this species. Eggs froth two-egg clutches were either equivalent in volume or larger than eggs from one-egg clutches. Thus, the egg quality hypothesis was rejected as an explanation for clutch size variation in the Nazca booby. Instead, two-egg clutches appear to be favored because of the insurance value of the second-laid egg, while one-egg clutches result from food limitation.