Contrasting estuary-scale distribution of wintering and migrating waders: the potential role of fear
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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.
KeywordsDunlin Calidris alpina Migration State-dependent habitat choice Predation risk Stopover Tagus estuary
We are indebted to many colleagues that kindly helped in wader catches, invertebrate sampling and laboratory analyses, mainly M Lecoq, I Catry, MP Dias, A Almeida, H Alonso and C Maldonado. Thanks are due to Reserva Natural do Estuário do Tejo for facilities made available for fieldwork and to Instituto da Conservação da Natureza e das Florestas for providing licenses to undertake captures and radio tag dunlins. We also acknowledge Fundação Gonçalves Júnior and Fundação das Salinas do Samouco for permission to undertake counts and wader catches in areas under their management. This study was funded by Fundação para a Ciência e Tecnologia through project Invisible Links (PTDC/MAR/119920/2010) and also by Grants to RCM (FRH/BD/44871/2008) and TC (SFRH/BPD/46967/2008 and SFRH/BPD/102255/2014).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interests.
Research involving human participants and/or animals
Neither dunlin captures nor the deployment of radio-tags caused any injury to the animals and both activities were performed in compliance with the Portuguese laws, under specific licenses provided by Instituto da Conservação da Natureza e das Florestas.
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