Variation in the seasonal pattern of mass changes by adult Arctic Skuas Stercorarius parasiticus on Foula, Shetland, was examined in relation to sex-role partitioning of breeding duties. Male body mass remained comparatively uniform throughout incubation and chick-rearing. By contrast, female mass showed an overall seasonal decline, although no significant changes occurred within incubation or chick-rearing. ANOVA confirmed a stepwise drop in female mass shortly after hatching, but that no further mass was lost during the remainder of the breeding season. We suggest that female Arctic Skuas retain body reserves during incubation as an insurance against natural fluctuations in food availability and/or to permit earlier relaying if the initial breeding attempt is unsuccessful. These reserves are then shed shortly after hatching as an adaptive response to reduce wing loading and improve flight efficiency in order to cope with the increasing energetic demands of growing chicks. The reduction after hatching of 4.3-5.3% of the female's incubation mass was estimated to result in energy savings of 5-10% of BMR in 24 hours, or alternatively, a reduction by 7-9% of the mechanical power required during flight.