The Collaborative International Dictionary of English v.0.48:

Yellowhammer \Yel"low*ham`mer\, n. [For yellow-ammer, where
   ammer is fr. AS. amore a kind of bird; akin to G. ammer a
   yellow-hammer, OHG. amero.] (Zool.)
      (a) A common European finch (Emberiza citrinella). The
          color of the male is bright yellow on the breast,
          neck, and sides of the head, with the back yellow and
          brown, and the top of the head and the tail quills
          blackish. Called also yellow bunting, scribbling
          lark, and writing lark. [Written also
          yellow-ammer.]
      (b) The flicker. [Local, U. S.]
          [1913 Webster]

WordNet (r) 3.0 (2006):

Emberiza citrinella
    n 1: European bunting the male being bright yellow [syn:
         yellowhammer, yellow bunting, Emberiza citrinella]