We measured daily energy expenditure (DEE) continuously for a whole year in a free ranging bird, the macaroni penguin Eudyptes chrysolophus. We combined these measurements with concurrently recorded foraging behaviour, and literature information on body mass and dietary factors to estimate prey consumption rates and foraging success. DEE was at a maximum during late chick-rearing but was equally high during all other active phases of the breeding season. DEE was approximately 4xresting metabolic rate, which accords with established theory and suggests a common 'energetic ceiling' throughout the summer period. However, whether this represents a maximum in physiological capacity, or a rate which optimises fitness is still unclear. Rates of prey consumption and foraging success followed different patterns from daily energy expenditure. Daily prey consumption was high as the penguins prepared for long fasts associated with moulting and incubation but relatively low during chick-rearing, when foraging areas were restricted and foraging success lower. It appears that the energy intake of macaroni penguins is subject to extrinisic or environmental constraints rather than to intrinsic physiological limits.