In wild populations, when settlement opportunities at the natal site are limited, some proportion of offspring undergo natal dispersal, that is, movement between the site of birth and the first breeding site. Dispersal is nearly ubiquitous in populations and it is a crucial driver of ecological and evolutionary patterns. Dispersal is assumed to have fitness costs, and short-term costs of dispersal incurred during the active dispersive phase are well documented. However, although information on immigrant (successful between-population dispersers) fitness is essential for our understanding of dispersal because of its major demographic and evolutionary consequences, information on the fitness costs incurred by immigrants is lacking, and the mechanistic processes underlying these costs remain almost entirely unknown. Major uncertainties in processes affecting immigrant fitness include a gap regarding our understanding of the performance of immigrants and residents (individuals remaining in their natal population) based on multiple trait approaches, a gap about the relative fitness of immigrants and residents in relation to sex, a gap regarding our understanding of the performance of immigrants and residents in long-lived species, and a gap in the nature of the behavioural and physiological mechanisms determinant of the fitness differences between immigrants and residents. Thus, to move forward, it is essential to understand the demographic and evolutionary consequences of immigrant fitness in wild populations, as well as the underlying mechanisms. ECOMIGR will carry out a comprehensive study of the demographic, behavioural and physiological mechanisms through which fitness of immigrants and residents differ. The project will exploit unique individual phenotypic and capture-recapture data, the latest miniaturized bio-logging technologies, physiological markers, state of the art analytical methods and modelling techniques, and unique long-term databases on the demography of seabirds. Seabirds breed on islands in colonies or in restricted habitat patches, which make them particularly suitable to study dispersal and facilitate the identification of immigrants in monitored populations. The PI has demonstrated the feasibility of quantifying and comparing the fitness costs between immigrant and resident individuals in long-lived species with a multiple demographic trait approach using detailed individual and longitudinal demographic data collected during several decades. By investigating the demographic, phenotypic, behavioural and physiological causes of fitness related differences between immigrants and residents in several long-lived species, and assessing to what extent immigrants and residents differ in their responses to environmental stochasticity, ECOMIGR will represent a major breakthrough in dispersal ecology and eco-evolutionary biology, as well as in conservation biology and the study of the effects of climate change on endangered seabirds.