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

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "ornithology":
   anatomy, animal physiology, anthropology, biology,
   comparative anatomy, conchology, ecology, entomology, ethology,
   helminthology, herpetology, ichthyology, malacology, mammalogy,
   protozoology, taxidermy, taxonomy, zoogeography, zoography,
   zoology, zoonomy, zoopathology, zoophysics, zootaxy, zootomy