Declines amongst breeding Eider Somateria mollissima numbers in the Baltic/Wadden Sea flyway

Abstract

We report on the status of the Baltic/Wadden Sea flyway Eider population based on trends in breeding and wintering numbers throughout the region, supplemented by changes in the sex ratio and proportion of young Eiders as monitored in the Danish hunting bag. At the flyway scale, total numbers of breeding pairs decreased by 48% during 2000–2009, after relatively stable breeding numbers in 1991–2000. The majority of the population nest in Finland and Sweden, where the number of breeding pairs has halved over the same period. After initial declines in winter numbers between 1991 and 2000, during 2000– 2009, national wintering numbers increased in the Baltic Sea, but decreased in the Wadden Sea. The annual proportion of adult females in the Danish hunting bag data de-Ornis Fennica 89:81–90. 2012 creased from ca. 45% (1982) to ca. 25% (2009) and simultaneously the proportion of first-winter birds fell from ca. 70% to ca. 30%, indicating dramatic structural changes in the Danish wintering numbers. These results suggest that the total flyway population will ex-perience further declines, unless productivity increases and the factors responsible for de-creasing adult female survival are identified and ameliorated. We discuss potential popu-lation drivers and present some recommendations for improved flyway-level monitoring and management of Eiders.