Introduction
The pigeon, Columba livia, belongs to the Aves or birds. Although birds have long been recognized as the flying, feathered vertebrates, recent fossil discoveries—among the more exciting paleontological developments of the last few decades—have revealed that feathers first evolved in several groups of theropod dinosaurs.
Feathers are therefore not unique to birds and can no longer be used to diagnose them as a group. Nonetheless, birds are the only living vertebrates with feathers. The terms Aves and birds have long been used to refer to the same group of amniotes, but they are not precisely synonymous. The Aves are considered the group that includes the ancestor of living birds and all of its descendants. However, the fossil remains of much more primitive birds have long been known to science.Indeed, birds have a long fossil record, beginning with the earliest bird, the Late Jurassic Archaeopteryx litho- graphica. The group including Archaeopteryx and all other birds is termed the Avialae and is recognized by development of the forelimbs into wings. The fossil record documents several substantial radiations during the Jurassic and Cretaceous Periods, of which many became extinct. As examples, the Confuciusornithidae (a group of toothless, flying birds that still had mainly separate digits in the manus and a pair of long ornamental tail feathers in, presumably, males), Hesperor- nithiformes (a group of flightless, diving birds with the forelimb represented only by a splint-like humerus), and Ichthyornithiformes (a group of ancient flying birds) may be mentioned. These groups had all become extinct by the end of the Cretaceous.
Modern birds, the Aves, began their radiation during the Cretaceous. They are among the most numerous of vertebrate groups, having diversified to include at least 9,700 living species (and molecular studies suggest that there might be twice as many species as currently recognized).
The Aves are characterized by complete loss of teeth and a very large sternal keel. They are subdivided into the Palaeognathae and Neognathae, which are differentiated primarily on palatal features. The paleognaths include two lineages, the flightless ratites (e.g., ostriches, emus, rheas, cassowaries) and the tinamous.Ratites and tinamous comprise some 54 living species, so most of present-day avian diversity is represented by the neognaths. Twelve main lineages are recognized, but their interrelationships are largely unresolved. Among these may be mentioned the sister groups Anseriformes (ducks, geese, swans) and Galliformes (chickens, grouse, pheasants, turkeys), which together apparently form the outgroup to other neognaths. The Procellariformes (albatrosses, petrels, and shearwaters), Pelicaniformes (pelicans, gannets, cormorants), Sphenisciformes (penguins), Gaviformes (loons), and Podicipediformes (grebes) are part of another lineage, as are the Falconi- formes (eagles, falcons, ospreys) and Strigiformes (owls). The Passeriformes, the great group of songbirds, together with Piciformes (barbets, toucans, woodpeckers) and Coraciiformes (hoopoes, hornbills, kingfishers) represent yet another lineage. The pigeon is included with doves in the Columbiformes, which is part of the lineage including Charadriiformes (auks, curlews, snipes, terns, gulls, puffins), Ardeidae (bitterns, egrets, herons), and Gruiformes (cranes, coots, rails, bustards).