The weight loss of incubating Black-browed Albatrosses (Diomedea melanophris) and Grey-headed Albatrosses (D. chrysostoma) was measured at Bird Island, South Georgia. The rate of weight loss did not differ significantly either between the sexes or between the species. The results suggest that these two species have the same metabolic requirements. The difference in the quality of their diet leads to estimates of daily food intake considerably higher for the Grey-headed than for the Black-browed albatross. This may have been a factor in the evolution of biennial breeding in the Grey-headed Albatross.