A co-operative project between France and Norway is proposed to study the physiological mechanisms (hormones and metabolic rate) involved in the regulation of parental effort (brood size) in an Arctic-breeding seabird, the kittiwake Rissa tridactyla. This project will be carried out at Kongsfjorden (Ny Ålesund, Svalbard) which constitutes one the northernmost (79° N) breeding site of the species. The main goal of this project is to understand the reasons of the very poor productivity of the species in this high-arctic area (only one chick/pair/year compared to 2-3 chicks/ pair/year in more temperate areas). To do so, we will concurrently study the metabolic cost of chick rearing and the metabolic cost of foraging. To test whether parent kittiwakes are apparently unable to rear more than one chick, we will manipulate brood size and will measure its consequences on basal metabolic rate (BMR) and foraging activity. We will experimentally manipulate the brood size by swapping chicks between nests shortly after hatching. Parent birds of the different experimental groups will be captured, weighted and a small blood sample (500 µL) will be taken for thyroid hormones. BMR will be estimated through thyroïd hormones (Chastel et al. 2003, J. Avian Biol. 34: 298-306), a method that reduces handling time imposed by the use of a respirometer, whereas activity at sea will be estimated using miniature activity recorders (Daunt et al., 2002 Mar. Ecol. Prog. Ser.245 : 239-247, Tremblay et al. 2003, J. Exp. Biol. 206: 1929-1940). Nests of the different groups (12 nests with 2 chicks and 12 nest with 1 chick) will be observed during 2 weeks after what parent birds will be recaptured, and bled again for T3 assay. On an other group of birds (N=10), we will calibrate these miniature activity recorders (N=10, weight:5 g) by observing the activities (rest, brooding, flying, etc..) of the instrumented birds in the colony. Food samples (N=12) will be collected from parent birds during capture and recapture sessions (kittiwakes spontaneously regurgitate food when handled). Breeding adults and chicks will be maked with plastic rings that allow identification from a distance.