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Mass loss of breeding birds might be due to the physiological stress of breeding or it could be an adaptation to lower the costs of flight to the feeding areas. We examined the natural variation in the adult body mass of Wilson’s storm petrels Oceanites oceanicus on King George Island, South Shetland Islands over four breeding seasons. During two seasons, the prey abundance was high, while it was poor during the other two seasons. Only breeding birds were sampled; the fluctuations in mass were similar among males and females. During incubation, the mass of the adults was high in good seasons and low in poor seasons. Thus, body mass during incubation was determined by energetic constraints. However, during chick feeding, adults lost mass in the good seasons but gained mass in the poor seasons, suggesting that mass loss during chick rearing is not primarily caused by stress, but is regulated adaptively. Adults in poor conditions may buffer against unpredictable food supply by increasing their own body mass, even at the expense of the chick. Reduced body condition at the beginning of the breeding season was associated with reduced egg volumes and late laying, suggesting that the initial body condition influenced the level of investment in the current breeding attempt. [ABSTRACT FROM AUTHOR]