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.
Classifications of birds following the traditional sequence of orders beginning, approximately, with Struthioniformes (ratites), Procellariformes (albatrosses, petrels), Sphenisciformes (penguins), and Gaviiformes (loons), and ending with Piciformes (woodpeckers) and Passeriformes (perching birds), as found in most field guides and checklists (e.g. Peters 1931-1951), are weakly connected to phylogenetic hypotheses, and tell as much about the history of ornithology as about the history of birds. A recent and revised classification of modern birds (Sibley and Monroe, 1990) reflects phylogenetic hypotheses, with sister groups being assigned coordinate ranks (following Hennig, 1966). However, while the approach is modern, this particular implementation rests on the problematic assumption that melting temperatures for hybridized DNA fragments from pairs of species can be extrapolated to accurately reflect divergence times.
Birds are unique in having feathers, which enable flight, provide insulation, and are used in visual communication. Modified feathers aid in swimming, sound production, protection via camouflage, water repellence, water transport, tactile sensation, hearing, and support of the body (Stettenheim, 1976). Birds are also warm-blooded, have distinctive bills, produce external eggs, and demonstrate complex parental and reproductive behaviors. Features shared with other reptiles, but not with mammals, include nucleated red blood cells, a single middle ear bone, and a single occipital condyle on the back of the skull. Adaptations for flight include fusion and reinforcement of lightweight bones and presence of a keeled sternum, which supports flight muscles. Birds have highly developed color vision, use vocalizations to mediate social interactions, and are able to detect and react to magnetism (see Gill, 1990).
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