Numbers and distribution of eiders somateria mollissima in the Wadden Sea

Abstract

Large numbers (in the order of 2·10 5) of eiders use the Wadden Sea as wintering, summering and moulting place and smaller numbers (∼10 3) breed here. The foreign birds originate from the Baltic Sea, where ∼10 6 eiders live. In January 1987, 245 900 eiders were counted in the Danish, German plus Dutch parts of the Wadden Sea. Similar numbers were observed in the seventies. Spring departures from the Wadden Sea last from February to the beginning of April. Males originating from the Baltic start to arrive in the Wadden Sea for moult in late May. Females moult nearer to their breeding grounds and somewhat later in the year. In July and August more than 80% of the eiders in the Danish and German parts of the Wadden Sea are males. Soon after moulting part of these males leave the area for the Baltic Sea to winter. The total numbers observed in the Wadden Sea in winter are not much higher than during moult. Numbers of moulting birds have increased dramatically in the German Wadden Sea since the seventies. In the Danish and Dutch Wadden Sea the number of moulters appear to be stable. Here the highest numbers are found in winter, while in the German Wadden Sea highest numbers are found in the moult period. The annual numbers of bird-days per km 2 ( viz. ∼10 4) do not differ much between the different parts of the Wadden Sea area. Total annual consumption by eiders in the Wadden Sea is estimated at 9·10 6 kg AFDW, which means a mean predation pressure of 1.2 g AFDW·m −2·y −1 ( i.e. a few percent of total benthic biomass). In the last few decades, the numbers of eiders more than doubled in the Baltic. They increased in the Wadden Sea during the moult period, but it is surprising that wintering numbers did not increase. Feeding conditions and relationships with mussel culture in the Wadden Sea are discussed. Overall distribution of eiders in the area is not clearly related to presence or absence of mussel culture plots, but the birds use these plots to some extent.