Labor division in parental care and reduction of inter-sex competition by division of the foraging niche have been suggested as part of the evolutionary basis for reversed sexual size dimorphism in boobies (Sulidae). To test the potential viability of both hypotheses for the maintenance of dimorphism, we studied parental care and foraging behavior of the Blue-footed Booby (Sula nebouxii) in two colonies in the Gulf of California, Mexico. Proportion of time spent at the nest, feeding frequency, and time devoted to nest defense were used as indicators of parental care. Foraging ecology was evaluated through the number and duration of foraging trips, proportion of time dedicated to flight and to active feeding, rate and depth of diving, and diet. We found some inter-sex differences in foraging only under high demand conditions, such as food shortage or large broods. Inconsistent inter-sex differences seem to be part of a strategy to maximize chick rearing under a variable environmental regime.

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