Merganser, also called Sawbill, fish duck, or trash duck, any of several species of Mergus, long-bodied, more or less crested diving ducks; though essentially freshwater birds, they are classified with scoters and goldeneyes in the sea duck tribe, Mergini (family Anatidae, order Anseriformes).
Mergansers are small ducks with a thin bill and a fan-shaped, collapsible crest that makes the head look oversized and oblong. In flight, the wings are thin and the tail is relatively long and rounded.
In eastern North America, many Mergansers move south and southwest in winter, but some actually migrate north to spend winters in the Great Lakes and southern Canada.