Feeding ecology of dunlins Calidris alpinastaging in the southern Baltic Sea, 1. Habitat use and food selection

The feeding habits of migrating dunlins Calidris alpina staging in different non-tidal coastal habitats in the southern Baltic Sea are described. The study also focuses on the structure of the benthic macrofauna of these habitats and the diet choice of dunlins. All investigations were carried out on Langenwerder Island (Wismar Bay), where different types of flats and beaches harbour a total of 30 to 40 species of marine macrofauna. The composition of the macrobenthos differed considerably between the eulittoral sandbank, the eulittoral mudflat, the pebble beach, and the sublittoral surroundings. Most dunlins were observed foraging in flocks of up to several hundred individuals on the eulittoral flats. Densities of up to 20 to 30 foraging dunlins ha-1 occurred annually during peak migration in September and October. Macrobenthos biomass in these habitats fluctuated between 20 and 40 g AFDM m-2. The mean total food consumption of dunlins during autumn migration was estimated at 0.01 g AFDM m-2 d-1. The predation pressure could be estimated at 3 to 6% of the suitable food supply. Dunlins staging on Langenwerder were able to attain a pre-migratory mass gain of 0.2 to 0.5% of their body weight per day within an 8 to 12-h daily feeding period. The birds fed predominantly on the polychaete Hediste diversicolor by probing. They selected small 7 to 31-mm-long individuals. When water levels were high, and the eulittoral flats inundated, many dunlins switched to foraging along the shorelines where a variety of small prey were taken from spilled macrophytes. Dunlins sometimes obviously ignored their most important food H. diversicolor, although available, by feeding on other prey such as juvenile fishes and shrimps, dipteran larvae or spilled amphipods. When feeding on amphipods, dunlins selected the smallest individuals.