| Abstract: | Estuarine mudflats, among the most important foraging grounds for waders during the non-breeding season, consist of complex mosaics of shallow pools and dry areas during low tide. In this study, we carried out close-range focal observations to determine foraging parameters of dunlins Calidris alpina, foraging in the mudflat microhabitats of the Tagus estuary, Portugal. Birds foraging in wet patches mostly targeted the siphons of the bivalve Scrobicularia plana , while in dry patches they mostly fed on mudsnails Hydrobia ulvae . Surface visibility of prey, rather than their abundance in the sediment, explained the microhabitat-related differences in prey selection. Birds using dry patches obtained 40% less energy intake than those using wet patches, still many extensively used this poor microhabitat. Because siphons retract quickly when the sediment is disturbed, birds often failed to catch them. We found that birds that were less efficient in capturing siphons in wet patches tended to spend more time foraging on mudsnails in dry patches. This suggests that lack of skills in siphon cropping represents a major foraging constraint for dunlins wintering in the Tagus estuary. It may even cause them to forage during high tide in order to achieve their daily energetic requirements. |
