Merganser n. (Zool.) 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.