Wintering and migrating populations of dunlin calidris alpina using the tagus estuary : foraging ecology, behaviour and distribution

Tese de doutoramento, Biologia (Ecologia), Universidade de Lisboa, Faculdade de Ciências, 2015Waders typically perform long-distance migrations between breeding and wintering areas, depending on a network of stopover sites to refuel along their flyway. Most wetlands act both as wintering and stopover areas for different wader populations and their distinct ecological constrains might lead them to different patterns of resource use, which is important information to plan conservation. The main aim of this thesis was to investigate the foraging ecology, behaviour and distribution of wintering and northward migrating dunlins Calidris alpina in the Tagus estuary, Portugal. These populations contrasted in diet, foraging behaviour and distribution. Differences in diet and foraging behaviour were explained by marked seasonal variations in prey availability. The improved feeding conditions for migrants presumably enabled high fattening rates, which highlight the good quality of the foraging habitats of the Tagus estuary for waders in stopover. In terms of distribution, unlike wintering dunlins, migrants avoided the areas of the estuary with higher perceived (but not real) risk of predation, despite their higher abundance of invertebrates. The distinct ecological constrains of these populations (e.g. in time and knowledge of the estuary) likely explains this pattern, as migrants are especially conservative in terms of predation risk when selecting foraging areas. In this thesis, the temporal overlap of wintering and migrating dunlins using the estuary in late winter/spring was investigated. This was done by determining departure dates of wintering birds, with radio-tracking, and through the innovative analysis of stable isotopes from toenails to unveil the wintering origins of birds using the estuary in migratory periods. This method confirmed the Banc d’Arguin, Mauritania, as the prime wintering area of birds that stopover at the Tagus estuary. To assist these studies, a new method to capture shorebirds was developed, where a crossbow is used to pull a mist-net over flocks of roosting birds