International audience Black-legged kittiwakes (BLKIs) reduce selfmaintenance cost through reductions in mass-speciWc basal metabolic rate (BMR), body mass and the size of visceral organs during the chick-rearing period. In the present study, we measured kidney in vitro oxygen consumption and plasma 3,3,5-triiodo-L-thyronine (T3) levels of incubating and chick-rearing female BLKIs, to test whether the decrease in BMR is caused mainly by decreased metabolic intensity or simply by reductions in the size of organs with high metabolic intensity. Body mass and body condition were lower in chick-rearing birds compared with the incubating birds. In contrast to the previous Wndings, however, the kidney mass did not diVer between the two breeding stages. Plasma T3 levels decreased substantially during the breeding season, indicating a reduction in BMR. Over the same period, kidney mass-speciWc oxygen consumption decreased (by 17.2%) from the incubating to the chick-rearing stage. Thus, the reduction in BMR found in breeding BLKIs seems partly explained by adjustments in metabolic intensity of visceral organs. Lowered metabolic intensity of visceral organs would permit increased allocation of energy to oVspring at the expense of their own self-maintenance.