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]

WordNet (r) 3.0 (2006):

Plectrophenax nivalis
    n 1: white Arctic bunting [syn: snow bunting, snowbird,
         snowflake, Plectrophenax nivalis]