any of several fish-eating, diving ducks with a long, slender, serrated bill hooked at the tip and, usually, a crested head, as the large (Mergus merganser) and the small (Lophodytes cucullatus)
See merganser in American Heritage Dictionary 4
(mər-gănˈsər)
noun
Any of various fish-eating diving ducks of the genus Mergus or related genera, having a slim hooked bill. Also called sheldrake.