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]

The Collaborative International Dictionary of English v.0.48:

Ring-necked \Ring"-necked`\, a. (Zool.)
   Having a well defined ring of color around the neck.
   [1913 Webster]

   Ring-necked duck (Zool.), an American scaup duck (Aythya
      collaris). The head, neck, and breast of the adult male
      are black, and a narrow, but conspicuous, red ring
      encircles the neck. This ring is absent in the female.
      Called also ring-neck, ring-necked blackhead,
      ringbill, tufted duck, and black jack.
      [1913 Webster]