The Collaborative International Dictionary of English v.0.48:

Scaup \Scaup\ (sk[add]p), n. [See Scalp a bed of oysters or
   mussels.]
   1. A bed or stratum of shellfish; scalp. [Scot.]
      [1913 Webster]

   2. (Zool.) A scaup duck. See below.
      [1913 Webster]

   Scaup duck (Zool.), any one of several species of northern
      ducks of the genus Aythya, or Fuligula. The adult
      males are, in large part, black. The three North American
      species are: the greater scaup duck (Aythya marila, var.
      nearctica), called also broadbill, bluebill,
      blackhead, flock duck, flocking fowl, and raft
      duck; the lesser scaup duck (Aythya affinis), called
      also little bluebill, river broadbill, and shuffler;
      the tufted, or ring-necked, scaup duck (Aythya
      collaris), called also black jack, ringneck,
      ringbill, ringbill shuffler, etc. See Illust. of
      Ring-necked duck, under Ring-necked. The common
      European scaup, or mussel, duck (Aythya marila), closely
      resembles the American variety.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Aythya affinis
    n 1: common scaup of North America; males have purplish heads
         [syn: lesser scaup, lesser scaup duck, lake duck,
         Aythya affinis]