Somateria mollissima, commonly known as the common eider, is native to the Northern Hemisphere, particularly found across northern coastal areas of Europe, North America, and eastern Siberia. Their geographical distribution includes countries such as Canada, the United States (especially Alaska), Greenland, Iceland, Norway, the United Kingdom, and Russia.

The common eider inhabits arctic and subarctic coastal marine environments. They are typically found in coastal lagoons, bays, and inlets that offer access to both open water and land for nesting. The species prefers areas with rocky coastlines, islands, and headlands where they can easily dive for food and find protection during breeding seasons.

Somateria mollissima is a marine duck that primarily feeds on crustaceans, mollusks, and small fish. Its ecological niche is as a benthic feeder, often diving to the ocean floor to forage for prey. During the breeding season, they nest on coastal islands or near the shorelines, using materials like vegetation and down to build their nests. The eider's ability to thrive in cold marine environments is facilitated by its thick plumage and layer of insulating fat, adaptations to conserving body heat in chilly waters.