Birds arose from theropod dinosaurs at some point in the Jurassic, according to present knowledge. Birds, together with the rest of the dinosaurs, the crocodilians, and their relatives are classified together in the Archosauria.
Living birds are classified in the taxon Neornithes. There are two subdivisions of the Neornithes, distinguished by details of palate anatomy: the Palaeognathae and the Neognathae. The Palaeognathae includes two subgroups: the Ratitae, which includes the ostrich, rhea, emu, and other large, cursorial, flightless birds (as well as the kiwi, which isn't so large); and the Tinamiformes, which includes the South America tinamous. All other living birds, from hawks to hummingbirds and from plovers to penguins, are classified in the Neognathae.
During the Mesozoic, several extinct lineages of birds shared the world with early Neornithes. The Ichthyornithiformes could fly and bore teeth in the jaws; known forms seem to have resembled gulls in their lifestyle, if not their structure. The Hesperornithiformes were also toothed, but most had small wings that were useless for flight (although a few hesperornithiforms that could probably fly have been found as fossils). Flightless hesperornithiforms seem to have swam by powerful foot propulsion, somewhat like modern cormorants. The most complex fossil bird taxon is the Enantiornithes. This group, which first appeared in the Lower Cretaceous, ranged from sparrow-sized birds to birds with wingspans a meter across, and included both toothed and toothless forms. Our knowledge of the Enantiornithes is still changing, and this taxon may turn out to be paraphyletic.
ITIS (Integrated Taxonomic Information System) contains a wealth of information on taxonomy and nomenclature. Visit ITIS to find out more about the taxonomy of Aves.