A rather bulky sea duck, weighing up to 2.5kg. The head of eider duck has a distinctive profile at all ages, with a large triangular bill that runs straight into the top of the head, giving a 'Roman nose' profile. In early summer the eider duck bird (male) is unmistakable, with a pattern of black on the crown, belly, flanks and tail, contrasting with the remainder of the body, which is white with a flush of emerald green on the hind-neck. The eider duck bird (females) are brown all over, with vertically barred flanks. Males in eclipse plumage are blackish-brown with white wing-patches and a white flank-spot. Maturity is reached only after three years, and first-year birds are strikingly plumaged with white breasts and scapulars on a blackish-brown body.
Mostly breeding above 55 degrees north, Eider duck birds are frequent shallow inshore coastal waters, often with small islands, rocks and skerries offshore. In winter they stay on the sea, typically dispersing just within their breeding range, wherever they can feed on crustaceans and molluscs, such as mussel beds. The species is accidental on inland waters.
The classic call of the eider duck bird (male) during courtship in spring and autumn is an almost comical "ah-OOO" or "oor-orWhooh", like the sound of appreciative coo-ing, often fusing into a conversation-like noise when multiple callers are in a flock together. The eider duck bird(female) has other calls, commonly heard when escorting chicks, such as a guttural "grrrar" and a "gog" note, commonly run together as a low grunting "og-og-og-o-og-og". The female also has a cawing "coo-roo" version of the male's call, given during display.