What is "fish duck"

Merganser \Mer*gan"ser\, n. [Sp. merg['a]nsar, fr. mergo a diver (L. mergus, fr. mergere to dip, dive) + ['a]nsar goose, L. anser.] (Zo["o]l.) Any bird of the genus Mergus ( Merganser), and allied genera of the subfamily Merginae. They are allied to the ducks, but have a sharply serrated bill, eat fish, and dive for food. Also called fish duck.

Note: The red-breasted merganser ( Merganser serrator) inhabits both hemispheres. It is called also sawbill, harle, and sheldrake. The American merganser ( Merganser Americanus.) and the hooded merganser ( Lophodytes cucullatus) are well-known species.

White merganser, the smew or white nun.

n. large crested fish-eating diving duck having a slender hooked bill with serrated edges [syn: merganser, sawbill, sheldrake]