Source : Webster's Revised Unabridged Dictionary (1913)

Snowbird \Snow"bird\, n. (Zo["o]l.)
   (a) An arctic finch ({Plectrophenax, or 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 {J. hyemalis} in the Eastern
       United States; -- called also {blue snowbird}. See
       {Junco}.
   (c) The fieldfare. [Prov. Eng.]

Source : WordNet®

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