The Collaborative International Dictionary of English v.0.48:

Redhead \Red"head`\ (-h?d`), n.
   1. A person having red hair.
      [1913 Webster]

   2. (Zool.)
      (a) An American duck (Aythya Americana) highly esteemed
          as a game bird. It is closely allied to the
          canvasback, but is smaller and its head brighter red.
          Called also red-headed duck. American poachard,
          grayback, and fall duck. See Illust. under
          Poachard.
      (b) The red-headed woodpecker. See Woodpecker.
          [1913 Webster]

   3. (Bot.) A kind of milkweed (Asclepias Curassavica) with
      red flowers. It is used in medicine.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Aythya americana
    n 1: North American diving duck with a grey-and-black body and
         reddish-brown head [syn: redhead, Aythya americana]