Extant birds include about 9000 recognized species, with representatives inhabiting all the major biogeographic regions of the world. Examples of bird groups and their native locales include: loons, auks and buntings in the Holarctic; rheas, motmots and toucans in the Neotropics; ostriches, guineafowl and woodhoopoes in Africa south of the Sahara; pheasants, pittas and babblers in Southeast Asia and northern Indonesia; and emus, cockatoos and owlet-frogmouths from Australia and New Guinea.

Whether modern birds are most closely related to dinosaurs or crocodylian ancestors is a point of current debate. The orders of extant birds appear to have arisen close to each other in time, although their age is uncertain, having been estimated to be about 60 million years old or over 90 million years old based on morphology and fossils (see Feduccia, 1996) and molecular data (Sibley and Ahlquist, 1990; Hedges et al., 1996), respectively.