The Collaborative International Dictionary of English v.0.48:

Goosander \Goos"an`der\, n. [OE. gossander, a tautological word
   formed fr. goose + gander. Cf. Merganser.] (Zool.)
   A species of merganser (M. merganser) of Northern Europe
   and America; -- called also merganser, dundiver,
   sawbill, sawneb, shelduck, and sheldrake. See
   Merganser.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sheldrake \Shel"drake`\, n. [Sheld + drake.]
   1. (Zool.) Any one of several species of large Old World
      ducks of the genus Tadorna and allied genera, especially
      the European and Asiatic species. (Tadorna cornuta syn.
      Tadorna tadorna), which somewhat resembles a goose in
      form and habit, but breeds in burrows.
      [1913 Webster]

   Note: It has the head and neck greenish black, the breast,
         sides, and forward part of the back brown, the
         shoulders and middle of belly black, the speculum
         green, and the bill and frontal bright red. Called also
         shelduck, shellduck, sheldfowl, skeelduck,
         bergander, burrow duck, and links goose.
         [1913 Webster]

   Note: The Australian sheldrake (Tadorna radja) has the
         head, neck, breast, flanks, and wing coverts white, the
         upper part of the back and a band on the breast deep
         chestnut, and the back and tail black. The chestnut
         sheldrake of Australia (Casarca tadornoides) is
         varied with black and chestnut, and has a dark green
         head and neck. The ruddy sheldrake, or Braminy duck
         (Casarca rutila), and the white-winged sheldrake
         (Casarca leucoptera), are related Asiatic species.
         [1913 Webster]

   2. Any one of the American mergansers.
      [1913 Webster]

   Note: The name is also loosely applied to other ducks, as the
         canvasback, and the shoveler.
         [1913 Webster]

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):

sheldrake
    n 1: large crested fish-eating diving duck having a slender
         hooked bill with serrated edges [syn: merganser, fish
         duck, sawbill, sheldrake]
    2: Old World gooselike duck slightly larger than a mallard with
       variegated mostly black-and-white plumage and a red bill