The Collaborative International Dictionary of English v.0.48:

Canvasback \Can"vas*back`\, n. (Zool.)
   A Species of duck (Aythya vallisneria), esteemed for the
   delicacy of its flesh. It visits the United States in autumn;
   particularly Chesapeake Bay and adjoining waters; -- so named
   from the markings of the plumage on its back.
   [1913 Webster]

WordNet (r) 3.0 (2006):

canvasback
    n 1: North American wild duck valued for sport and food [syn:
         canvasback, canvasback duck, Aythya valisineria]