There is significant evidence that birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.

The basal bird Archaeopteryx, from the Jurassic, is well-known as one of the first "missing links" to be found in support of evolution, though it is not considered a direct ancestor of modern birds.The recently discovered dromaeosaur Cryptovolans was capable of powered flight, possessed a sternal keel and had ribs with uncinate processes. In fact, Cryptovolans makes a better "bird" than Archaeopteryx which is missing some of these modern bird features. At any rate, it is fairly certain that avian flight existed in the mid-Jurassic and was "tried out" in several lineages and variants by the mid-Cretaceous.

Modern birds are classified in Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous (see Vegavis). The Neornithes are split into the Paleognathae and Neognathae. The paleognaths include the tinamous (found only in Central and South America) and the ratites. The ratites are large flightless birds, and include ostriches, cassowaries, kiwis and emus (though some scientists suspect that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages). The basal divergence from the remaining Neognathes was that of the Galloanseri, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (the pheasants, grouse, and their allies). See the chart for more information.

The classification of birds is a contentious issue. Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). A preponderance of evidence seems to suggest that the modern bird orders constitute accurate taxa. However, scientists are not in agreement as to the relationships between the orders; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.