During this period of annual feather moult energy demands are high, making them potentially vulnerable to reduced prey availability and severe weather conditions. ![]() They forage by pursuit diving and the medium and larger sized species undergo moult of their flight and body feathers outside the breeding season (hereafter referred to as during winter). ![]() Members of the auk family (Alcidae) dominate the avian community wintering in the North Sea. However, the challenge of quantifying foraging distribution and diet of sympatric species of mobile predators means that this assertion remains largely untested. ![]() The ability of sympatric species with similar ecological niches to respond to environmental variation, and the relative nature of their responses, could profoundly influence patterns of species distribution, particularly when environmental disturbances occur during critical life history stages. The nature of any such response reflects a balance of risk/reward associated with resource intake, energetic costs and physiological demands. Mobile predators may potentially respond to changes in prey availability by moving to find new resources or switching diets. How animals respond to changing environmental conditions is a key topic in movement and foraging ecology. Conservation of mobile predators, many of which are in sharp decline, may benefit from dynamic spatial based management approaches focusing on behavioural changes in response to changing environmental conditions, particularly during life history stages associated with increased mortality. Populations of North Sea puffins and razorbills showed contrasting foraging responses when environmental conditions, as indicated by overwinter survival differed. By contrast, razorbills’ trophic position increased in the poor survival winter and the population foraged in more distant southerly waters of the North Sea. However, puffin diet significantly differed, with a lower average trophic position in the winter characterised by lower survival rates. Puffins foraging in the North Sea used broadly similar foraging locations during moult in both winters. Puffins and razorbills showed divergent foraging responses to contrasting winter conditions. We used a combination of bird-borne data loggers and stable isotope analyses to test 1) whether these sympatric species showed consistent responses with respect to foraging location and trophic position to these contrasting winter conditions during periods when body and cheek feathers were being grown (moult) and 2) whether any observed changes in moult locations and diet could be related to the abundance and distribution of potential prey species of differing energetic quality. MethodÄifferences in overwinter survival rates of two populations of North Sea seabirds (Atlantic puffins ( Fratercula arctica) and razorbills ( Alca torda)) indicated that environmental conditions differed between 2007/08 (low survival and thus poor conditions) and 2014/15 (higher survival, favourable conditions). However, in marine environments this assertion remains largely untested for highly mobile predators outside the breeding season because of the challenges of quantifying foraging location and trophic position under contrasting conditions. ![]() In order to persist, sympatric species with similar ecological niches may show contrasting foraging responses to changes in environmental conditions. Higher trophic level consumers may respond to changes in the distribution and quality of available prey by moving to locate new resources or by switching diets. Natural environments are dynamic systems with conditions varying across years.
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