Roosting waterbirds at the Osterems, German wadden Sea: seasonal and spatial trends studied by aerial and ground surveys

Abstract

Waterbirds migrating through and wintering in the Wadden Sea depend on undisturbed roost sites close to their feeding areas. Birds at these remote places are, however, often not counted within standard monitoring programmes. As in the Osterems a new remote sandbank island, the Kachelotplate, is emerging, we initiated a pilot study based on aerial counts to evaluate the importance of this island as roost site for waterbirds in the context of the surrounding islands. To check for the representativeness of the aerial counts, we compared numbers (i) on one occasion with a synchronous ground survey, and (ii) all aerial counts with the long-term average for those areas regularly covered by ground counts. Numbers of birds in flocks counted during synchronous aerial and ground counts were in good agreement. Nevertheless, birds in structured areas and small waders were difficult to count from the air. In general, a calibration of aerial counts and long-term experience is necessary to obtain realistic results by counting waterbirds from an aeroplane. Waterbirds in the Osterems showed the typical seasonal phenology for the East-Frisian Wadden Sea. Given the size of the study area, bird numbers were relatively high. Besides the Kachelotplate, which was important for the Common Eider and gulls for roosting, another remote island, Lutje Horn, was of paramount importance as roost site, in particular for the Eurasian Oystercatcher, Eurasian Curlew, and in spring for terns. A clear distinction could be made between outer, open islands like the Kachelotplate, which were used by birds feeding on pelagic food sources, and sheltered areas, used by benthic foragers, supporting the idea that birds try to reduce travelling costs between feeding and roosting areas. Furthermore, the study underlined the importance of remote, undisturbed roosting areas for waterbirds in the Wadden Sea.