We investigated how the annual variation in the abundance of wintering common eiders
Somateria mollissima was related to the abundance of benthos over 4 yr. The study was conducted at
2 sites, Tunø Knob and Ringebjerg Sand, Denmark, from 1994 to 1998. Benthos was sampled along
permanent transects in November, and common eiders counted and localized from observation
towers during the winter. The benthic community was dominated by the blue mussel Mytilus edulis,
with sizes varying with depth. Small and intermediary-sized individuals were primarily found in 0 to
6 m depth, and large individuals exclusively in 6 to 12 m depth. The pattern of variation in abundance
of common eiders matched that of the benthos in 0 to 6 m depth, where they were mostly found, but
not in 6 to 12 m depth. Based on a bioenergetic model of food consumption, we estimated that common
eiders during the 3rd and 4th years exploited between 22 and 64% of the entire benthic community
biomass in 0 to 6 m depth, compared to ²3% in 6 to 12 m depth. We conclude that common
eiders tracked annual variation in food abundance, primarily related to the presence of small and
intermediary-sized blue mussels.