The Collaborative International Dictionary of English v.0.48:

Ornithology \Or`ni*thol"o*gy\, n. [Ornitho- + -logy: cf. F.
   ornithologie.]
   1. That branch of Zoology which treats of the natural history
      of birds and their classification.
      [1913 Webster]

   2. A treatise or book on this science.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ornithology
    n 1: the branch of zoology that studies birds