Ornithology is the branch of zoology that deals with the study of birds. Birds are any organisms in the class Aves. They are warm-blooded (or homoiothermic) vertebrates that have feathers covering their body; forelimbs modified into wings; stouter hindlimbs used for walking, swimming, or perching; scaly legs and feet; jaws reduced to a toothless beak; and a four-chambered heart. Birds lay hard-shelled eggs from which their young hatch. Major subject areas in ornithology include: anatomy, physiology, behavior, ecology, evolution, and classification and systematics.
Birds evolved from a group of reptiles known as the dinosaurs (order Dinosauria), which first appeared during the late Triassic period (which ended about 210 million years ago). The earliest bird known in the fossil record is Archaeopteryx, from the mid-Jurassic period about 160 million years ago. These extremely early birds had many typical avian characteristics, such as feathers and a horny beak, and were very likely warm-blooded, but they also had reptilian features, such as teeth. In fact, modern birds and dinosaurs still have many characteristics in common, and some biologists and paleontologists believe that birds should be viewed, and classified, as "living dinosaurs."
There are 27 orders of birds, divided into about 166 families, and containing about 9,000 living species. Some prominent families from the Americas include the Gaviidae (loons), Podicipedidae (grebes), Pelecanidae (pelicans), Phalacrocoracidae (cormorants), Anatidae (swans, geese, ducks, mergansers), Cathartidae (vultures), Accipitridae (hawks, eagles), Falconidae (falcons), Tetraonidae (grouse, ptarmigan), Phasianidae (quail, pheasants), Ardeidae (herons, bitterns), Rallidae
(rails, coots), Charadriidae (plovers, turnstones), Scolopacidae (sandpipers), Laridae (gulls, terns), Columbidae (pigeons, doves), Strigidae (owls), Trochilidae (hummingbirds), Psittacidae (parrots, macaws), Picidae (woodpeckers), Tyrannidae (flycatchers), Hirundinidae (swallows), Corvidae (jays, crows, raven), Paridae (chickadees, titmice), Troglodytidae (wrens), Turdidae (thrushes, bluebirds), Sturnidae (starlings), Vireonidae (vireos), Parulidae (wood warblers), Icteridae (blackbirds, orioles), and Fringillidae (grosbeaks, finches, sparrows).
Avian anatomy changes remarkably during development, from the relatively simple structures of the fertilized egg, to the much more complex adult form. Anatomy is also extremely variable among species. Size alone ranges from flightless ostriches weighing up to 330 lb (150 kg), to tiny hummingbirds weighing only 0.08 oz (2.25 g). Perhaps the most distinguishing anatomical characteristic of birds is their feathers, which provide insulation against the loss of body heat, and a broad, yet light, wing and tail surface for flight. The forelimbs of birds are highly modified as wings, especially through the extension of their "fingers" into an airfoil surface for active and/or gliding flight (a few species, such as the ostrich and emu, have secondarily lost the power of flight). Other important anatomical adaptations for flying include the large breast muscles that are used to power flight, the large keeled sternum (or breastbone) to which the flight muscles attach, and the hollow bones and extensive air-sacs of most species of birds. Some birds have extraordinarily light bodies for their size—the magnificent frigatebird (Fregata magnificens) has a wingspan of 7 ft (2 m), but its skeleton weighs only 4 oz (113 g), less than the weight of its feathers.
Birds lay eggs with a hard shell that contain a fertilized embryo. In most species, the eggs are laid into a nest constructed by one or both of the parents, and the eggs are incubated by the parents. The newly hatched young of most bird species are born in a relatively early stage of development, and are unfeathered, ungainly, and virtually helpless. These almost incompetent young must be fed and otherwise tended by their parents for some time, until they finish development and learn to fly and forage for themselves. The young of some other species are more developed when born, and may be capable of immediately leaving the nest to live a semi-independent, or even fully independent life.
A distinguishing element of avian physiology is homoiothermy, or metabolic activity that maintains the body temperature within a narrow, warm range optimized for muscular functions and enzyme efficiency. (Mammals and some other vertebrates are also warm-blooded.) Homoiothermy is a crucial physiological trait that allows almost all bird species to have an extremely active lifestyle. It also allows some species to live year-round in cold environments.
One of the most notable elements of avian behavior is their use of song to proclaim a breeding territory, and other distinct sounds to organize their social system, keep flocks together, warn other individuals of predators, and for other kinds of communication. Some other classes of animals are also rather vocal, particularly many mammals, but not to the same degree that most birds are. Some birds are also quite intelligent, second in this regard only to mammals. (Actually, corvids such as the raven Corvus corax are more intelligent than most species of mammals.) Another notable element of avian behavior is the habit of many species to undertake long-distance migrations between their breeding and wintering habitats. The most extensive migrations are made by the arctic tern (Sterna paradisaea), some of whom breed at the most northerly limits of land on the northern tips of Greenland and Ellesmere Island, and then migrate to spend their winter foraging in Antarctic waters.