It is well known that energy metabolism is strongly related to body size and internal organ masses. At the subcellular level, studies have clearly reported that interspecific variation in body mass can be related to some aspects of mitochondrial bioenerge tics. In birds, the reproductive period is a highly energy demanding period that usually involves reallocation of energy from self-maintenance and survival of adults to investment in egg-formation, incubation and growth of the chicks. In this context, the reproductive success of an individual would depend upon good management of the energy budget in order to ensure the current survival of offspring and the maintenance of body condition of the adults. Since mitochondria are the main cells powerhouses, prov iding cells with energy in the form of ATP, mitochondrial functioning must play an important role in the successfulness of an animals breeding season and beyond in the animals general fitness and life history trajectory. However, little, if anything, is k nown about the mitochondrial energetic adjustments during breeding seasons and its potential importance in the reproductive success of birds. In the present project we test if there are cellular mechanisms responsible for the decrease in metabolic rate, w hich is observed in the black-legged kittiwake (Rissa tridactyla) from incubation to the chick-rearing period. We hypothesize that the efficiency of mitochondrial oxidative phosphorylation would increase during the chick-rearing period, shifting the mitoc hondrial metabolism towards an economic mode of functioning