Anthropogenic change paved the way for a traditionally arctic-breeding, long-distance migratory bird, the barnacle goose to stop migrating and also breed successfully in temperate regions. This provides us with the unique opportunity to compare migratory and non-migratory individuals of originally the same population, and to quantify costs and benefits of migration. In our mechanistic approach to ecology we will focus on the energetic consequences and the pace-of-life of migratory versus non-migratory lifestyles. We will do so by tracking free-ranging individuals representing both lifestyles, and quantify their rates of metabolic turnover at high temporal resolution and throughout the annual cycle. From the integration of individual-based, behavioural data (movement and activity patterns) with physiological measurements (heart rate, body temperature) and environmental data (weather, time, place, habitat) we can identify energy-conserving mechanisms, and the conditions under which these are employed in natural settings. We will determine rates of organismal ageing by longitudinal studies of telomere (i.e., repetitive nucleotide sequences at the end of chromosomes) shortening during juvenile growth and adulthood. Aside from a fundamental interest in how animals adopt new lifestyles, the proposed project will also generate valuable data for an evidence-based, adaptive management of one of the fastest-growing waterfowl populations in the world.