The Collaborative International Dictionary of English v.0.48:

Snowbird \Snow"bird\, n. (Zool.)
   (a) An arctic finch (Plectrophenax nivalis syn.
       Plectrophanes nivalis) common, in winter, both in
       Europe and the United States, and often appearing in
       large flocks during snowstorms. It is partially white,
       but variously marked with chestnut and brown. Called also
       snow bunting, snowflake, snowfleck, and
       snowflight.
   (b) Any finch of the genus Junco which appears in flocks in
       winter time, especially Junco hyemalis in the Eastern
       United States; -- called also blue snowbird. See
       Junco.
   (c) The fieldfare. [Prov. Eng.]
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Junco \Jun"co\, n. (Zool.)
   Any bird of the genus Junco, which includes several species
   of North American finches; -- called also snowbird, or
   blue snowbird.
   [1913 Webster]

WordNet (r) 3.0 (2006):

snowbird
    n 1: medium-sized Eurasian thrush seen chiefly in winter [syn:
         fieldfare, snowbird, Turdus pilaris]
    2: white Arctic bunting [syn: snow bunting, snowbird,
       snowflake, Plectrophenax nivalis]
    3: small North American finch seen chiefly in winter [syn:
       junco, snowbird]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "snowbird":
   LSD user, acidhead, addict, alcoholic, chain smoker,
   cocaine sniffer, cokie, cubehead, dipsomaniac, dope fiend, doper,
   drug abuser, drug addict, drug user, drunkard, fiend, freak,
   glue sniffer, habitual, head, heavy smoker, hophead, hype, junkie,
   marijuana smoker, methhead, narcotics addict, pillhead, pothead,
   speed freak, tripper, user