The Collaborative International Dictionary of English v.0.48:

Plumage \Plum"age\ (pl[=u]m"[asl]j), n. [F., from plume a
   feather.] (Zool.)
   The entire clothing of a bird.
   [1913 Webster]

   Note: It consist of the contour feathers, or the ordinary
         feathers covering the head, neck, and body; the tail
         feathers, with their upper and lower coverts; the wing
         feathers, including primaries, secondaries, and
         tertiaries, with their coverts; and the down which lies
         beneath the contour feathers. See Illust. under Bird.
         [1913 Webster]

WordNet (r) 3.0 (2006):

plumage
    n 1: the light horny waterproof structure forming the external
         covering of birds [syn: feather, plume, plumage]