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Modern birds are a class of vertebrates characterized by being feathered, warm-blooded (endothermic), and bipedal (two-legged); and by having very high metabolic rates and a forelimb modified into a wing which, together with a long tail, forms part of a flight mechanism. Such a definition, however, as with any group of vertebrates, characterizes the living forms and is blurred by the fossil record, which contains species with characteristics close to those of the reptilian ancestors of birds. The feathers of birds are filamentous, lightweight modifications of the outer skin that have remarkable aerodynamic qualities. They serve not only as flight structures by generating lift and thrust but also as insulation to maintain high body temperatures. In addition, birds have lightweight hollow bones, a well-developed air-sac system and flow-through lungs, a wishbone or furcula (fused clavicles), and a hand reduced to three digits (comparable to digits 2, 3, and 4 of the human hand). Birds are known to have evolved from some group of reptiles within the larger group of ancient diapsid reptiles known as archosaurs. However, debate still centers on whether they are derived from a common ancestor with the theropod (meat-eating) dinosaurs (a group known as basal archosaurs), or later in time directly from theropod dinosaurs.

Feathers are unique to birds. These lightweight structures made of keratin are the most complex appendages produced by the skin of any vertebrate. The bird wing comprises two sets of flight feathers, the outer primary feathers which are attached to the hand, and the inner secondary feathers which are attached to the ulna. The vanes of the wing feathers are asymmetric with a smaller outer vane and larger inner vane, producing lift in flight. The body feathers provide small aerodynamic contours which result in laminar airflow in flight. Most of the vane is stiff and tightly bound, like flight feathers. However, the basal portion can by fluffed up to trap body heat next to the skin. In warm conditions the body feathers can be flattened to allow heat to escape. Thus, feathers form an insulatory pelt to cover the surface of the avian body. The tail feathers resemble the flight feathers of the wing and provide lift in flight. The tail feathers of modern birds are attached to a specialized bone known as the pygostyle, which is formed by a number of fused tail vertebrae. The pygostyle (sometimes called the plowshare bone) also accommodates the uropygial gland, or oil gland, an essential part of the anatomy of birds that provides a rich waterproofing oil for preening the feathers. In addition to their primary functions of flight and insulation, feathers can serve other functions, ranging from the production of color patterns and structural forms that allow for species recognition and courtship displays, to color patterns that serve a cryptic purpose for protection. See Feather

The once-toothed jaws of birds have evolved into the lighter beak in which the upper and lower jaws are covered by a horny rhamphotheca, which may vary in texture from the rock-solid beaks of predatory raptors to relatively soft beaks of shorebirds and ducks. Beaks have a great variety of adaptive forms, including the flesh-tearing hooked beaks of hawks and eagles, the filter-feeding straining beaks of flamingos and ducks, the fish-trapping beaks of pelicans, the climbing and nut-cracking beaks of parrots, the hammering beaks of woodpeckers, and the seed-eating beaks of finches. Birds have developed a muscular gizzard (also found in their relatives, crocodiles and dinosaurs) for grinding and processing food into small pieces. The grinding is often assisted by the addition of gizzard stones, which are ingested.

Birds have varied feet. The most primitive avian foot, found in the earliest known bird, Archaeopteryx, is the perching foot also found in most modern tree-dwelling birds. Three toes point forward, and a reversed first toe, or hallux, opposes them in perching on a branch. This type of foot is called anisodactyl. Other modifications include the zygodactyl feet of woodpeckers with two forward and two rearward pointing toes, and the webbed feet of ducks with the three forward pointing toes united by a web that serves as a paddle. The varied birds in the order Pelecaniformes have a foot in which all four toes are united by webbing, a totipalmate foot. Ostriches are unique in the bird world in having a foot with only two toes. The ankle and foot bones are fused and elongated in birds, so that the avian leg consists of a femur, tibiotarsus, and fibula, then a fusion of three bones into a tarsometatarsus, and finally the toes. Thus, birds walk on their toes, and the equivalent to the human foot is a long bone, the tarsometatarsus, which is off the ground.

Birds have keen senses of vision and hearing. The sense of smell (olfaction) is not particularly well developed, although in some birds there is a good sense of smell. Birds have developed a flow-through lung and an extensive air-sac system. Modern flying birds have a well-developed sternum with a keel, or carina, for the attachment of the large flight musculature.

Birds are found over the entire Earth. One of the most intriguing aspects of bird biology is the ability to migrate exceptional distances. Birds possess highly specialized directional senses for orientation, navigation, homing, and migration, including the ability to detect the Earth's magnetic field. These uncanny abilities permit birds to occupy distinctive wintering and nesting grounds, thus expanding their usable habitats. Some migrations, such as that of the Arctic tern, involve a circumatlantic migration from Alaska to the South Pole. See Flight

There are some 9700 species of birds living today, and most species are particularly well known. However, the relationships of the higher categories of birds are still debated. Of the 9700 species, some 5000 species belong to the order Passeriformes, the perching birds or songbirds. The number of avian orders is still controversial, and texts show different arrangements. Because the situation is in flux, a fairly conservative system is used below (fossil groups are designated by a dagger).

  • Class Aves
  • Subclass Sauriurae
  • Infraclass Archaeopterygiformes
  • Order Archaeopterygiformes (late Jurassic reptile-birds)
  • Order Confuciusornithiformes (lower Cretaceous, beaked reptile-birds)
  • Infraclass Enantiornithes (archaic Mesozoic land birds)
  • Subclass Ornithuriae
  • Infraclass Odontornithes (or Odontoholcae)
  • Order Hesperornithiformes (Cretaceous toothed divers)
  • Infraclass Neornithes (or Carinata)
  • Superorder Ambiortimorphae (gull-like, Mesozoic toothed birds)
  • Incertae sedis (Gansus, Chaoyangia, etc., archaic modern-type birds)
  • Palaeognathae (ostrich and allies)
  • Neognathae (modern birds)
  • Order: Sphenisciformes (penguins, 17 species)
  • Procellariiformes (tube-nose seabirds, 114)
  • Pelecaniformes (pelicans and allies, 66)
  • Ciconiiformes (storks and allies, 86)
  • Falconiformes (hawks, eagles, and vultures, 309)
  • Galliformes (chickens and allies, 282)
  • Gruiformes (rails, cranes and allies, 214)
  • Podicipediformes (grebes, 22)
  • Charadriiformes (shorebirds, gulls and allies, 349)
  • Pteroclidiformes (sand grouse, 16)
  • Threskiornithiformes (ibis, spoonbills, 33)
  • Anseriformes (waterfowl, 161)
  • Phoenicopteriformes (flamingos, 5 or 6)
  • Gaviiformes (loons, 5)
  • Columbiformes (pigeons and doves, 316)
  • Psittaciformes (parrots, 360)
  • Coliiformes (mousebirds, 6)
  • Musophagiformes (turacos, or plaintain-eaters, 23)
  • Cuculiformes (cuckoos, 142)
  • Opisthocomiformes (hoatzin, 1)
  • Strigiformes (owls, barn owls, 173)
  • Caprimulgiformes (nightjars and allies, 116)
  • Apodiformes (hummingbirds and swifts, 425)
  • Trogoniformes (trogons, quetzals, 39)
  • Coraciiformes (kingfishers, bee-eaters and allies, 219)
  • Piciformes (woodpeckers and allies, 407)
  • Passeriformes (perching birds, songbirds, passerines, 5739)

The classification system presented above coordinates with most major treatises on birds. The subclass Sauriurae contains the archaic birds of the Mesozoic Era, the Age of Reptiles, which includes the toothed fossil Archaeopteryx, or Urvogel, the oldest known bird (150 million years ago). Other Mesozoic birds included the ancient ornithurine birds more closely allied with the modern radiation of birds, among them such forms as the hesperornithiforms, the Cretaceous toothed divers, which superficially resembled loons. They became extinct at the end of the Cretaceous along with their gull-like contemporaries, the Ambiortimorphae. Also included in this group is the Lower Cretaceous Ambiortus from Mongolia, which was a fully volant ornithurine bird about the size of a pigeon. It possessed a well-developed sternal keel and other features of the pectoral region typical of modern birds, indicating that true flying birds existed some 12 million years after the appearance of Archaeopteryx.

The extinction of the dinosaurs 65 million years ago is now believed to have been due to the collision of a large extraterrestrial body, a meteor or some other object, with Earth, causing catastrophic effects, including the extinction of numerous bird species. It is likely that the very few types that survived, possibly related to shorebirds, were the wellspring of the modern evolution of birds; and modern birds, like their mammalian counterparts, probably evolved explosively during the early part of the Tertiary Period, perhaps over a period of some 5–10 million years. Among the first birds to appear were the strange Diatrymas, large, predatory, flightless birds that had a head the size of that of a horse. They are thought to have taken over the niche left vacant by predatory dinosaurs, and they fed on the small archaic mammals of the Paleocene and Eocene.

By the Eocene, approximately 50 million years ago, all the major orders of modern birds were present. By the Oligocene, most of the families were present, and by the Miocene, the genera of modern birds were well established.

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(birds), a class of vertebrates that are closely related to reptiles in origin and certain structural features; birds and reptiles are united in the superclass Sauropsida.

Birds are bipedal terrestrial egg layers that are adapted to flight. (Penguins, ostriches, and some other birds have lost the ability to fly.) The skeleton is light and sturdy. The forelimbs have been converted into wings, and the body is covered with feathers, which reduce heat loss. The heart has four chambers, with complete separation of arterial and venous blood. Metabolism is intensive, and the body temperature is constant and high (37.8°–5.5°C).

Structural features and their function. The skin of birds is thin and in two layers; it lacks glands, including sweat glands. Most birds have only an oil, or uropygial, gland. The skin is covered with down and feathers. The remiges and rectrices serve for flight, and the covert feathers cover certain parts of the body, known as the pterylae. The bare spaces between the feathered parts are called apteria. In a few birds, such as ostriches, cassowaries, and penguins, the feathers evenly cover the entire body. The plumage color is determined by pigments (melanins and lipochromes), by structural colors of the feather (which create a metallic sheen), or by both. Many birds experience color changes during certain parts of the sexual cycle or in a particular season. The plumage and the horny coverings of the bill and legs are molted regularly, usually once a year. Molting may be complete or partial.

The skeletal structure of birds is conditioned by adaptations to flight, when the wings hold the entire weight of the body in the air, and to walking or climbing, when the load falls on the limbs of the pelvic girdle. The bones, which are pneumatic, are rigidly connected or, in many cases, completely fused; there is a high content of lime salts in the bone tissue. These characteristics ensure that the skeleton is light and sturdy. The thin-walled brain case of the skull lies behind the large orbits, which are separated by a thin septum. The toothless jaws are sheathed with a horny covering and form a bill. The cervical region of the vertebral column consists of 11 to 25 vertebrae and is freely movable. The thoracic vertebrae, which are partially fused to the backbone, bear the ribs, three (or nine) pairs of which are joined to the sternum. Each rib consists of two parts that articulate at an angle. This makes it possible to change the volume of the thorax during intensive respiration. Avian ribs also have hooklike appendages that increase the strength of the thorax. The sacral vertebrae and the bones of the pelvic girdle, which are fused, provide sturdy support for the legs, which serve for walking, climbing, or swimming, as well as for grasping prey. The posterior caudal vertebrae are fused together to form the pygostyle, to which the tail feathers are attached.

The pectoral girdle is formed by the scapulae, clavicles (which usually fuse to form the wishbone, or furcula), the coracoids, and the sternum. In most birds, the sternum has a keel, or carina, which serves as an attachment for the flight musculature. The wing skeleton is formed by the humerus, the forearm (ulna and radius), and the hand (with two metacarpal bones, a carpometacarpus, and toes with a reduced number of phalanges). The flight feathers attached to the hand are called primaries; those attached to the forearm are the secondaries. The skeleton of the legs is formed by the femur, tibia, and metatarsal bone; there are four toes or, less frequently, three (in the ostrich, two).

The musculature, especially the flight musculature, is highly developed. Thus, the muscles that participate in flight (chiefly the pectoralis) may constitute 11 (coots) to 50 percent (tinamous) of the body weight. These muscles are located close to the bird’s center of gravity to ensure stability in flight. The leg musculature is also well developed.

Differences in skeletal and muscular structure determine the characteristics of flight in different birds. Soaring birds, such as albatrosses, storks, and predatory birds, fly with minimal expenditure of muscular energy and make use of upward air currents. Flapping flight requires that much strength be expended on energetic motion of the wings. Soaring birds have an elongated humerus, whereas birds marked by more active flight (hummingbirds, swifts) generally have a greatly shortened humerus. A pheasant loses its principal energy on vertical (explosive) takeoff and has relatively more powerful pectoral musculature than, for instance, a seagull, which frequently soars in the air.

The avian respiratory system is quite unique. The bronchi, which lead to the small, poorly expandable lungs, are joined to the system from nine or ten elastic air sacs. During voluntary or involuntary inspiration (in the course of wing flapping), air passes through the lungs into the air sacs; during expiration, the air sacs release the air, sending it through the lungs and thus increasing the intensity of gas exchange. The air sacs also regulate body temperature and, in aquatic birds, help to change the density of the body during diving.

Rapid blood circulation is ensured by the relatively large volume of the heart, especially in small birds, and by the frequency of the heartbeats. In hummingbirds the heart constitutes up to 2.85 percent of the total body weight. Ostriches have a heart rate of 60 to 70 beats per minute, and hummingbirds up to 1,000 beats per minute.

The avian esophagus sometimes has an enlargement, known as the crop, where food is temporarily stored and undergoes preliminary chemical processing. In the proventriculus (the thin-walled anterior glandular section of the stomach) the food is broken up by digestive juices. In the muscular gizzard (the posterior section of the stomach) the food is thoroughly ground. The gizzard of birds that feed on grain is particularly muscular. Digestion is completed in the small intestine, where there are pancreatic and liver secretions, and in the ceca (these are absent in parrots and predatory birds), which open between the small and large intestines. The large intestine leads to the cloaca. Digestion is intensive. Undigested bones, fur, fish scales, and insect chitins are regurgitated by many birds in the form of pellets.

The avian bird is relatively large and has well-developed cerebral hemispheres. The cortex is underdeveloped, and its associative functions are performed by the hyperstriatum. The optic lobes and the cerebellum are also well developed. The medulla oblongata is only slightly separate from the spinal cord, which has cervical and lumbosacral enlargements from which the nerves to the wings and legs depart. Vision, hearing, and equilibrium are well developed in birds, but olfaction and taste are not. The eyeballs are very large and only slightly mobile in their orbits (in owls they are immobile). The resulting limitation of the field of vision, especially in birds with frontal eye placement (owls and buzzards), is compensated by the mobility of the neck. When there is highly developed vocal communication, hearing plays a significant role in a bird’s life. The perfect hearing apparatus of owls permits them to find their prey easily, even in the dark. Oilbirds and salanganes, which nest in dark caves, orient themselves by means of echolocation.

The organs of excretion—kidneys—are very large. There is no urinary bladder, and the ureters open into the cloaca, where some of the water contained in the urine is absorbed by the walls, decreasing the body’s need for water. The male sex organs consist of two testes, which enlarge greatly before the breeding season. The deferent ducts depart from the testes and lead into the cloaca. Only a few birds—the more primitive ones (for example, ostriches and geese)—have a copulatory organ. The female sex organs usually consist of a left ovary and an oviduct which opens into the cloaca. (Owls and falconiform birds also have a right ovary.) The fertilized egg in the upper section of the oviduct is covered with an albuminous casing; it is subsequently enclosed in shell membranes and, finally, in a calcareous shell. In the lowest section of the oviduct, the egg acquires the characteristic color of its group. Unfertilized eggs with thin shells are laid by birds that have eaten insects, rodents, and seeds contaminated with pesticides.

Button quails attain sexual maturity in four or five months; many passerines, pigeons, and ducks in ten or 11 months; swifts, many seagulls, and geese in two years; eagles, vultures, and storks in four to six years; and the albatross Diomedea epomophora in eight years.

Reproduction occurs cyclically with the seasonal development of the sex glands, which is influenced by internal (hormonal) factors and such external factors as an increase in the number of daylight hours, the onset of the rainy season in the tropics, and changes in feeding conditions. Food deficiency disrupts the reproductive cycle: owls and falconiform birds do not mate when rodent populations are small, and crossbills do not mate when there are no coniferous seeds. There may be one, two, or three reproductive cycles per year. The sites chosen for nesting are quite varied, ranging from tropical forests to coastal land floes in the antarctic (emperor penguin). There are also various types of nests—from pits in the ground to the cleverly coiled nests of tits and Icteridae. The number of eggs per clutch varies from one (Procellariiformes, many guillemots, lyrebirds) to 20 or more (Galliformes). The duration of incubation is strictly defined for each group of birds. The eggs may be incubated by only the females (ducks, Galliformes), only the males (button quails, painted snipes), or both sexes. Megapodes incubate their eggs on rotting forest floor or volcanic sand. Nidicolous hatchlings are born blind and helpless, remaining in the nest for a long time and needing warmth and feeding. Nidifugous birds are sighted, leaving the nest and feeding independently immediately after hatching. The life-span of birds in captivity is 15 to 20 years for small birds, up to 52 years for the white pelican, up to 56 years for the sulphur-crested cockatoo, and up to 70 years for the eagle owl. Under natural conditions, the life-spans are much shorter.

Birds are distributed from the arctic to the shores of the antarctic. They are encountered in all natural zones—from arctic wastelands to tropical forests—and at all elevations—from sea level to the alpine zone. Some birds lead a sedentary life. Many, however, with deterioration of conditions of existence (onset of winter, drought, lack of forage plants) and under the influence of internal factors, change their habitats to more favorable ones, making long or short migrations.

Origin. The remains of fossil birds are rare, since their brittle bones do not preserve well. The ancestors of birds are thought to be the reptiles Pseudosuchia—in particular, the family Euparkeria—which moved on their hind limbs and are known from Lower Triassic deposits. Imprints of feathers and remains of three skeletons of Archaeopteryx, which had feathers yet retained many reptilian traits, have been found in Jurassic deposits.

Taxonomy. The class of Aves is usually divided into 28 extant orders: Sphenisciformes, Struthioniformes, Rheiformes, Casuariiformes, Apterygiformes, Tinamiformes, Gaviiformes, Podicepediformes, Procellariiformes, Pelecaniformes, Ciconiiformes, Phoenicopteriformes (flamingos—often assigned to Ciconiiformes), Anseriformes, Falconiformes, Galliformes, Gruiformes, Charadriiformes, Columbiformes, Psittaciformes, Cuculiformes, Strigiformes, Caprimulgiformes, Apodiformes, Coliiformes, Trogoniformes, Coraciiformes, Piciformes, and Passeriformes. Some ornithologists adopt a more detailed classification, distinguishing as many as 40 orders. For example, the order Charadriiformes is divided into three orders—the gulls, guillemots, and snipes, and the order Gruiformes is broken into nine orders. The division of Aves into two superorders —Ratitae and Carinatae—is no longer accepted. The total number of bird species is approximately 8,600, including more than 5,000 passerine species. About 750 bird species are encountered in the USSR.

Environmental changes (logging, reclamation of swamps, plowing of virgin lands), pollution, and unregulated hunting have led to a sharp decrease or the complete decimation of the population of many bird species. Many flightless island birds (dodoes, some water rails) disappeared with the importation of pigs, rats, and mongooses. Since the early 16th century, about 170 bird species have become extinct, and approximately 300 more species are in danger of disappearing. Endangered species in the USSR include the red-breasted goose, white crane, whitenaped crane, hooded crane, Manchurian crane, and little bustard. The woodpecker Picus squamata and the crested ibis no longer nest in the USSR.

The importance of birds in nature and for man is great and diverse. Birds have been hunted for a long time. Some have been domesticated and have been developed into numerous breeds of chickens, ducks, turkeys, geese, and pigeons. The beautiful appearance and song of birds enliven forests, orchards, and parks. Birds pollinate plants and disseminate seeds, thus distributing both valuable plants (oak, cedar, oil palm) and weeds. Birds are valued by man because they eat destructive insects and rodents and the seeds of weeds. At the same time, some bird species damage plantings, orchards, and vineyards; destroy useful insects; and dismember nests and kill the nestlings of game birds. They also carry certain diseases (vultures and crows transmit malignant anthrax; sparrows, fowl cholera; and parrots and pigeons, ornithoses) or act as an intermediate link in the transmission of tree viruses. With the development of jet aviation, aircraft accidents caused by collision with birds have increased in frequency, especially near airports.

The branch of zoology that studies birds is called ornithology.


Menzbir, M. A. Ptitsy. St. Petersburg, 1904‣09.
Shul’pin, L. M. Ornitologiia. Leningrad, 1940.
Dement’ev, G. P. Ptitsy. Moscow-Leningrad, 1940. (Rukovodstvo po zoologii, vol. 6.)
Ptitsy Sovetskogo Soiuza, vols. 1‣6. Edited by G. P. Dement’ev and N. A. Gladkov. Moscow, 1951‣54.
Fauna SSSR: Ptitsy, vol. 1 (issues 2‣5), vol. 2 (issues 1 and 3). Moscow-Leningrad, 1937‣65. (AN SSSR: Zoologicheskii Institut. Novaia seriia, nos. 14, 19, 30, 33, 65, 80, 81, 91.)
Zhizn’ zhivotnykh, vol. 5. Moscow, 1970.
Peterson, R. Ptitsy. Moscow, 1973. (Translated from English.)
Stresemann, E. von. Sauropsida: Aves. Berlin-Leipzig, 1927‣34. (Handbuch der Zoologie, vol. 7, part 2. Compiled by W. Kükenthal.)
Biology and Comparative Physiology of Birds, vols. 1‣2. New York-London, 1960‣61.
A New Dictionary of Birds. Edited by A. F. Thomson. London, 1964.
Avian Biology, vols. 1‣2. New York-London, 1971‣72.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


(vertebrate zoology)
A class of animals composed of the birds, which are warm-blooded, egg-laying vertebrates primarily adapted for flying.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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