Diet of eiders and body condition change from the late 1980s to the mid 2010s

Abstract

Food and foraging decisions are fundamental drivers of the population dynamics of bird species. Eider Somateria mollissima was sampled in Danish marine waters during the non-breeding season in 1986–1990 and in 2010–2017 in order to analyze changes in diet and body condition between the two periods. The food items found were among those commonly consumed by eiders. Between the late 1980s and the mid 2010s there was a decrease in the number of blue mussels Mytilus edulis, cockles Cerastoderma edule and common whelks Buccinum undatum in the diet, and an increase in the number of razor clam Ensis directus. Blue mussel was the most abundant prey. When present, this prey item occurred almost exclusively in the eiders' digestive tract and was negatively correlated with the numbers of other prey items. Eiders with blue mussels, other bivalves and shore crabs in the gizzard had better body condition than eiders with other food items. Blue mussel was a preferred food item and is important for the build-up of body condition. Body condition increased between the late 1980s and the mid 2010s. We assume that this increase in body condition has occurred in parallel with a decrease in the size of the population of the Baltic/Wadden Sea flyway.