The common eider, also called St. Cuthbert's duck or Cuddy's duck, is a large sea-duck that is distributed over the northern coasts of Europe, North America and eastern Siberia. It breeds in Arctic and some northern temperate regions, but winters somewhat farther south in temperate zones, when it can form large flocks on coastal waters. It can fly at speeds up to 113 km/h (70 mph).

The sea ducks (Mergini) are a tribe of the duck subfamily of birds, the Anatinae. The taxonomy of this group is incomplete. Some authorities separate the group as a subfamily, while others remove some genera. Most species within the group spend their winters near coastal waters. Many species have developed specialized salt glands to allow them to tolerate salt water, but these are poorly developed in juveniles. Some of the species prefer riverine habitats. All but two of the 22 species in this group live in far northern latitudes.