The Collaborative International Dictionary of English v.0.48:

Yellowbird \Yel"low*bird`\, n. (Zool.)
   (a) The American goldfinch, or thistle bird. See Goldfinch.
   (b) The common yellow warbler; -- called also summer
       yellowbird. See Illust. of Yellow warbler, under
       Yellow, a.
       [1913 Webster]

WordNet (r) 3.0 (2006):

yellowbird
    n 1: yellow-throated American wood warbler [syn: yellow
         warbler, golden warbler, yellowbird, Dendroica
         petechia]
    2: American finch whose male has yellow body plumage in summer
       [syn: New World goldfinch, goldfinch, yellowbird,
       Spinus tristis]