The Collaborative International Dictionary of English v.0.48:

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.] (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.
   [1913 Webster +PJC]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Lophodytes cucullatus
    n 1: small North American duck with a high circular crest on the
         male's head [syn: hooded merganser, hooded sheldrake,
         Lophodytes cucullatus]