Two hypotheses have been proposed to explain sex-related differences in foraging in sexually monomorphic seabirds. The ‘intersexual competition’ hypothesis suggests that parents are in competition and that the dominated sex must adapt its behaviour to avoid competing with the dominant one. The ‘energetic constraint’ hypothesis suggests that differential energetic requirements lead to different foraging behaviour. We examined sexual differences in foraging behaviour of a sexually monomorphic tropical seabird, the Barau’s petrel, Pterodroma baraui, throughout the breeding period. We found sexual differences in foraging habitats and activities, but these were not consistent throughout the breeding period. During the prelaying exodus, males foraged further from the colony, in waters with greater surface chlorophyll concentration, and were more active than females. Males systematically took the first incubation shift, which was always longer than the others. However, no sex-related differences in foraging behaviour were observed during the chick-rearing period, both sexes sharing parental duties equally. We suggest that the sexual differences observed early in the breeding period are due to the specific needs of males and females. Females need to restore their body condition as quickly as possible after laying, which forces males to take the first long incubation shift at the nest. This may explain why males forage more actively during the prelaying exodus, to prepare for this long fasting period. Our results support for the first time the ‘energy constraint’ hypothesis to explain sexual differences in behaviour of a small, monomorphic procellariiform seabird.