Merganser, any bird of the genus Mergus, of the duck family, with six species from the Palasarctic and Nearctic regions, Brazil, and the Auckland Islands. The bill is straight and slender, hooked at the tip, and strongly toothed along the edges. Besides the Goosander (q.v.), the Red-breasted Merganser (M. senator) breeds in Scotland, the Smew (M. albellus), sometimes called the Nun from its black-and-white plumage, visits Britain in severe winters, and the Hooded Merganser (M. cucullatus) is a rarer visitor. M. brasiliensis is from South America, and M. australis from the Auckland Islands.