Resource landscapes explain contrasting patterns of aggregation and site fidelity by red knots at two wintering sites

Background: Space use strategies by foraging animals are often considered to be species-specific. However,similarity between conspecific strategies may also result from similar resource environments. Here, we revisit classicpredictions of the relationships between the resource distribution and foragers’ space use by tracking free-livingforagers of a single species in two contrasting resource landscapes. At two main non-breeding areas along theEast-Atlantic flyway (Wadden Sea, The Netherlands and Banc d’Arguin, Mauritania), we mapped prey distributionsand derived resource landscapes in terms of the predicted intake rate of red knots (Calidris canutus), migratorymolluscivore shorebirds. We tracked the foraging paths of 13 and 38 individual red knots at intervals of 1 s overtwo and five weeks in the Wadden Sea and at Banc d’Arguin, respectively. Mediated by competition for resources,we expected aggregation to be strong and site fidelity weak in an environment with large resource patches. Theopposite was expected for small resource patches, but only if local resource abundances were high.Results: Compared with Banc d’Arguin, resource patches in the Wadden Sea were larger and the maximum localresource abundance was higher. However, because of constraints set by digestive capacity, the average potentialintake rates by red knots were similar at the two study sites. Space-use patterns differed as predicted from thesedifferences in resource landscapes. Whereas foraging red knots in the Wadden Sea roamed the mudflats in highaggregation without site fidelity (i.e. grouping nomads), at Banc d’Arguin they showed less aggregation but werestrongly site-faithful (i.e. solitary residents).Conclusion: The space use pattern of red knots in the two study areas showed diametrically opposite patterns.These differences could be explained from the distribution of resources in the two areas. Our findings imply thatintraspecific similarities in space use patterns represent resp