Body condition of Eiders at Danish wintering grounds and at pre-breeding grounds in Åland

Breeding Eiders Somateria mollissima in the Baltic/Wadden Sea population have declined in recent decades, and several causes have been suggested. A recent study showed a modest increase in body mass from 2.20 kg by ca. 100 g during winter in Denmark, the main wintering area of the flyway population. This raises the question of whether Eiders could increase their body mass by an additional 200 g during pre-breeding to reach the average body mass of ca. 2.50 kg needed for successful reproduction. We analyzed this question by collecting Eiders at the Danish wintering grounds and pre-breeding grounds at land, Finland. Female Eiders increased their body condition (defined as scaled body mass index) from winter to the pre-breeding period, whereas males concurrently decreased in weight. The mean body mass of adult female Eiders was 2.23 kg in late winter, when they initiated spring migration. The mean female body mass at the pre-breeding grounds was 2.44 kg, an increase of 9.4% compared with females at the wintering grounds, while adult males decreased in weight by 3.4%. A decrease in gizzard mass was accompanied by a change in diet from large blue mussels Mytilus edulis at the wintering grounds to the small but more abundant mussels in the gizzard during pre-breeding. We conclude that female Eiders can reach a body mass of ca. 2.50 kg before breeding. Building up body condition for breeding is feasible at land; thus, difficulty in acquiring adequate resources for reproduction is unlikely to restrict local population size.