Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. A treatise or book on this science.

Source : WordNet®

ornithology
     n : the branch of zoology that studies birds