Migratory species spend different parts of the annual cycle in geographically disparate places but events and processes occurring at each stage are inextricably linked because what occurs in one season carry over to influence individual success the following season. For instance, habitat quality in the non-breeding grounds can affect physical condition, reproductive success and survival during the breeding season; and similarly, conditions and reproductive performance during breeding can also influence migration strategy and the chances of survival in the subsequent non-breeding season. Since carryover effects are likely to be responsible for a large amount of variation in individual fitness, assessing their importance is critical for understanding population dynamics and implementing conservation and management strategies, but it has only been described in a few terrestrial species. Within this framework, we will first determine migratory ecology of a given complex of seabird species (Calonectris sp) breeding on Mediterranean and Atlantic Islands, at inter-specific, intra-specific, and individual levels. Secondly, and more interestingly, we will assess the influence of seasonal interactions and the carryover effects on individual fitness and population dynamics in these highly pelagic seabird species. To test these seasonal links, we will study empirically and experimentally the movements of shearwaters (including foraging effort and wintering areas), annual schedules (arrival/departure dates), moulting patterns, breeding performance, physical condition, levels of stress, habitat quality and survival throughout the year. Thus, by combining biologging techniques, spatial modelling, biogeochemistry, animal physiology, and environmental ecology, in addition to observational data, this proposal will enable the applicant to develop his training in the fields of animal behaviour and migratory ecology and to obtain original results of broad basic and applied relevance.