A chunky, short-billed, buoyant sea duck; dark-winged, blunt-tailed except in adult male, with a confusing variety of patterns. Typically has round head with pale eye-patch and dark cheeks. It dives constantly. Social, often with scoters, but stray individuals turn up inland and in estuaries.

Long-tailed ducks live in flocks, often far offshore , and frequently associate with scoters and Eiders. The flocks are very active, often flying fast over the sea, and can resemble auks as they whirr along, flashing black and white and rolling between the wavetops.  In a few areas they move close inshore, especially at the mouths of estuaries, where they are brought in by the tides.