Contrasting estuary-scale distribution of wintering and migrating waders: the potential role of fear

In estuaries hosting both wintering and migrating populations of waders of the same species, the distinct ecological constraints on birds in different seasons may result in different criteria being used for selection of suitable foraging habitat. We analysed the distribution patterns of dunlins Calidris alpina in the Tagus estuary, Portugal, during the non-breeding season and investigated the roles of prey availability and predation risk to explain those patterns. The southern estuary provided higher prey availability but their narrower flats may induce greater fear of predation in waders than the open northern flats. However, our data suggest that the real risk was similar. Migrating birds avoided the southern estuary, favouring areas perceived as safer over better feeding opportunities. In contrast, wintering dunlins favoured the southern flats, despite their proximity to cover. Presumably, wintering waders have a better knowledge of the estuary, including its real predation risks, taking advantage of the best foraging areas. Without such knowledge, waders in short stopovers have to select their foraging areas based on indirect indicators of predation risk, such as distance to cover. This study illustrates the importance of incorporating specificities of habitat preferences by wintering and migrating wader populations in conservation planning for large estuaries.