any of the 9,600 living species unique in having feathers (feather), the major characteristic that distinguishes them from all other animals. A more elaborate definition would note that they are warm-blooded vertebrates more related to reptiles than to mammals, having a four-chambered heart (as do mammals), forelimbs modified into wings (a trait shared with bats), a hard-shelled egg, and keen vision, the major sense they rely on for information about the environment. Their sense of smell is not highly developed, and auditory range is limited. Most birds are diurnal in habit. More than 1,000 extinct species have been identified from fossil remains.

Since earliest times birds have been not only a material but also a cultural resource. Bird figures were created by prehistoric man in the Lascaux Grotto of France and have featured prominently in the mythology and literature of societies throughout the world. Long before ornithology was practiced as a science, interest in birds and the knowledge of them found expression in conversation and stories, which then crystallized into the records of general culture. Ancient Egyptian hieroglyphs (hieroglyph) and paintings, for example, include bird figures. The Bible refers to Noah's use of the raven and dove to bring him information about the proverbial Flood. Various bird attributes, real or imagined, have led to their symbolic use in language as they did in art. Aesop's fables abound in bird characters. The Physiologus and its descendants, the bestiaries (bestiary) of the Middle Ages, contain moralistic writings that use birds as symbols for conveying ideas but indicate little knowledge of the birds themselves. Supernatural beliefs about birds probably took hold as early as recognition of the fact that some birds were good to eat. Australian Aborigines for example, drove the black-and-white flycatcher from camp, lest it overhear conversation and carry the tales to enemies; peoples of the Pacific Islands saw frigate birds (frigate bird) as symbols of the Sun and as carriers of omens and frequently portrayed them in their art; the raven—a common symbol of dark prophecy—was the most important creature to the Indians of the Pacific Northwest and was immortalized in Edgar Allan Poe's poem The Raven. Eagles (eagle) have long been symbols of power and prestige in many parts of the world, including Europe, where their representations are often seen in heraldry; Native Americans sprinkled eagle down before guests as a sign of peace and friendship, and eagle feathers were commonly used in rituals and headdresses. The resplendent quetzal—the national bird of Guatemala, the name of its currency, and a popular motif in art, fabric, and jewelry—was worshipped and deified by the ancient Mayans and Aztecs. Highly symbolic birds include the phoenix, representing resurrection, and the owl, a common symbol of wisdom but also a reminder of death in Native American mythology. The bird in general has long been a common Christian symbol of the transcendent soul, and in medieval iconography a bird entangled in foliage symbolized the soul embroiled in the materialism of the secular world.

In modern times the recreational pleasures of bird-watching have grown in tandem with the rise of environmentalism. Evolving from the American and European “shoot-and-stuff” mania of the 19th century, bird-watching became a sportlike activity based on rapid identification—the rarest being the most rewarding—with the aid of binoculars and spotting scopes. The change from shooting to sighting coincided with campaigns, beginning about 1900, to halt the slaughter of wild birds for food and millinery. Bird-watching was advanced by the publication of excellent field guides and improvements in photography and sound recording. By mid-century the watcher's enjoyable but rather unsophisticated tallying of “year lists” and “life lists” of species personally observed was being augmented, if not replaced, by interest in careful studies of bird behaviour, migration, ecology, and the like. This trend was abetted by bird banding (called ringing in the United Kingdom) and by such organizations as the British Trust for Ornithology and the National Audubon Society, which coordinate professional and amateur observations and efforts with scientific studies.

Birds arose as warm-blooded, arboreal, flying creatures with forelimbs adapted for flight and hindlimbs for perching. This basic plan has become so modified during the course of evolution that in some forms it is difficult to recognize.

The ability to fly has permitted an almost unlimited diversification of birds, so that they are now found virtually everywhere on Earth, from occasional stragglers over the polar ice caps to complex communities in tropical forests. In general the number of species found breeding in a given area is directly proportional to the size of the area and the diversity of habitats available. The total number of species is also related to such factors as the position of the area with respect to migration routes and to wintering grounds of species that nest outside the area. In the United States, Texas and California have the most—approximately 620 for each (the figure varies based on criteria used for inclusion on state lists, such as unconfirmed, accidental, hypothetical, extirpated, and extinct species). More than 920 species have been recorded from North America north of Mexico. The figure for Europe west of the Ural Mountains and including most of Turkey is 514. More than 700 species live in Russia. At least 4,400 species live in North and South America. Although several South American countries boast well over 1,000 species, Costa Rica, with an area of only about 51,000 square km (about 20,000 square miles) and a known avifauna of more than 800 species, probably has the most diversity for its size. Asia accounts for more than 25 percent of the world's species, with 2,700 species, and Africa slightly less, with about 2,300.

In addition to their importance in literature and legend, birds have been significant to human society in myriad ways. Birds and their eggs (egg) have been at least incidental sources of food for humans since their origin and still are in most societies. The eggs of some colonial seabirds, such as gulls, terns, and murres (murre), or guillemots (guillemot), and the young of some muttonbirds (muttonbird) are even now harvested in large quantities. With the development of agrarian human cultures, several species of chickens (chicken), ducks (duck), geese (goose), and pigeons (pigeon) were taken in early and have been selectively bred into many varieties. These domestic birds are descended, respectively, from the red jungle fowl (Gallus gallus), mallard duck (Anas platyrhynchos), greylag goose (Anser anser), and rock dove (Columba livia). After the discovery of the New World, the turkey (Meleagris gallopavo), which had already been domesticated by the Indians, and the Muscovy duck (Cairina moschata) were brought to Europe and produced several varieties. guinea fowl (Numida meleagris) from Africa were also widely exported and kept not only for food but also because they are noisy when alarmed, thus warning of the approach of intruders.

With the development of modern culture, hunting evolved from a foraging activity to a sport, in which the food value of the game became secondary. Large sums are now spent annually on hunting waterfowl, quail, grouse, pheasants (pheasant), doves (dove), and other game birds. Sets of rules and conventions have been set up for hunting, and in one elaborate form of hunting, falconry, there is not only a large body of specialized information on keeping and training falcons but also a complex terminology, much of it centuries old.

Feathers (feather) have been used for decoration for many thousands of years. Their use in the headpieces of indigenous peoples throughout the world is well known. Feather robes were made by Polynesians and Eskimos; and down quilts, mattresses, and pillows are part of traditional European folk culture. Large feathers have often been used in fans, thereby providing an example of an object put to opposite uses—for cooling as well as for conserving heat. Whereas most feathers used in decorating are now saved as by-products of poultry raising or hunting, until early in the 20th century, egrets (egret), grebes (grebe), and other birds were widely shot for their plumes alone. Ostrich farms have been established to produce plumes as well as meat, and some ostriches have been raised specifically for racing. Large quills were once widely used for writing, and feathers have long been used on arrows and fishing lures.

Many birds are kept as pets (pet). Small finches (finch) and parrots (parrot) are especially popular and easy to keep. Of these, the canary (Serinus canaria) and the budgerigar of Australia (Melopsittacus undulatus, often called a parakeet) are widely kept and have been bred for a variety of colour types. On large parks and estates, ornamental species such as peacocks (peacock) (Pavo cristatus), swans, and various exotic waterfowl and pheasants are often kept. Zoological parks in many cities import birds from many lands and are a source of recreation and enjoyment for millions of people each year.

With the rise of agriculture, man's relationship with birds became more complex. Vast quantities of guano (bird excrement) were mined from island breeding colonies for use as fertilizer for crops. However, in regions where grain and fruit are grown, depredations by birds may be a serious problem. In North America various species of blackbirds (blackbird) (family Icteridae) are serious pests in grainfields; in Africa a grain-eating finch, the red-billed quelea (Quelea quelea), occurs like locusts, in plague proportions so numerous that alighting flocks may break the branches of trees. The use of city buildings for roosts by large flocks of starlings (starling) and blackbirds is also a problem, as is the nesting of albatrosses (albatross) on airplane runways on Pacific islands. As a result of these problems, conferences on the control of avian pests (pest) are commonly held.

The study of birds has contributed much to both the theoretical and practical aspects of biology. Charles Darwin's (Darwin, Charles) studies of the Galapagos finches (Galapagos finch) and other birds during the voyage of HMS Beagle were important in his formulation of the idea of the origin of species through natural selection. Collections of birds in research museums still provide the bases for important studies of geographic variation, speciation, and zoogeography, because birds are one of the best known of animal groups. Early work on the domestic fowl added to the development of both genetics and embryology. The study of animal behaviour ( ethology) has been based to a large extent on studies of birds by Konrad Lorenz (Lorenz, Konrad), Nikolaas Tinbergen (Tinbergen, Nikolaas), and their successors. Birds also have been the primary group in the study of migration and orientation and the effect of hormones on behaviour and physiology.

Because of their body structure and their feathered covering, birds are the best fliers among animals, better than insects and bats. There are, however, considerable differences in ability among various birds. Penguins (penguin) cannot fly, instead spending much of their time in the water swimming with their paddlelike wings. Birds such as ostriches (ostrich) and emus (emu) have rudimentary wings but are permanently afoot. At the other extreme, long-winged swifts (swift) and frigate birds (frigate bird) move from their perches only to fly, never to walk. Most birds alternate some walking or swimming with their flying.

Birds usually fly when they have any considerable distance to travel; there are exceptions, however. The mountain quail of California make their annual migrations up and down the mountains on foot. The guillemots (guillemot) of the Greenland coast migrate southward by swimming; they begin their journey before the young have grown their flight feathers and before some of the adults at least have regrown their recently molted ones. The Adélie penguins may ride northward on drifting ice floes; at the approach of nesting time, they swim back to the Antarctic continent and then walk over the ice to their breeding grounds many miles inland.

Birds fly by flapping their wings, steering mainly with their tails. Compared to the parts of an airplane, a bird's wing acts as both wing and propeller. The basal part of the wing supplies most of the supporting surface, the wing tip most of the propelling force. A bird's wing has many adjustable features: it can be shortened or lengthened by flexion; the feathers of the tip can be spread or closed; the angle of the whole wing or its parts—on one side or both—can be altered. All of these adjustments make the aerodynamics of a bird's wing much more complicated than those of the airplane; consequently, the flight of a bird is much more varied and adaptable.

The types of flight found in birds vary considerably, and different types of wings correlate with different types of flight. At least two major types of modifications for gliding or soaring are found. Albatrosses (albatross) and some other seabirds have long, narrow wings and take advantage of winds over the oceans, whereas some vultures (vulture) and hawks (hawk) have broad wings with slotted tips that permit more use of updrafts and winds deflected by hills. Short, broad wings are characteristic of chickenlike birds, which fly up with a rush of rapid wing beats. Birds such as ducks, pigeons, and falcons, which fly rapidly with continuous wing beats, tend to have moderately long, pointed wings. Many songbirds (songbird) use their short, rounded wings to move with quick wing beats from perch to perch or from ground to perch. Ducks (duck) have pointed wings that, beaten at high speed, provide rapid flight for long distances. Swallows (swallow), terns (tern), and frigate birds have long, pointed wings that enable these birds to fly and maneuver gracefully for hours with leisurely wing beats. Large herons (heron) with long, broad wings travel far with slow, measured strokes, while buzzards (buzzard) soar high in the sky on their long, broad wings. Gulls and albatrosses, with their long, narrow wings flapping infrequently, sail along the beaches or over the waves. Swifts and hummingbirds, with their narrow, curved wings, fly rapidly and maneuver easily. A hummingbird can whir its tiny wings so rapidly that it can hover as it thrusts its long bill into a blossom; it can even fly backward as it leaves the bloom.

● 15–30 km/hr (kilometres per hour) (10–20 mph 【miles per hour】)—many small songbirds such as sparrows (sparrow) and wrens (wren)

● 30–50 km/hr (20–30 mph)—many medium-sized birds such as thrushes (thrush) and grackles (grackle), and larger, long-winged birds such as herons (heron), pelicans (pelican), and gulls

● 30–60 km/hr (20–40 mph)—many small- and medium-sized birds such as starlings (starling), chimney swifts, and mourning doves (mourning dove)

● 60–100 km/hr (40–60 mph)—the faster-flying birds such as falcons (falcon), ducks, geese (goose), and domestic pigeons. A homing pigeon has been timed at 152 km/hr (94 mph).

● The fastest bird, however, is the peregrine falcon, whose speed in a dive has been measured in excess of 320 km/hr (200 mph).

The record long-range flight of a bird species in a single season is undoubtedly held by the Arctic terns (tern) that migrate from a summering ground in the Arctic to a wintering ground in the Antarctic, travelling more than 11,600 km (7,200 miles) each way. Some long-range flights are made very quickly: a blue-winged teal banded in Canada was recovered 6,100 km (3,800 miles) away in Venezuela only 30 days later; a Manx shearwater, trapped at its nest in Wales and transported 5,200 km (3,200 miles) to Massachusetts and released, returned home in 12¹/₂ days. Some very small birds regularly make long water crossings in a single flight. Ruby-throated hummingbirds (hummingbird) fly across the more than 800-km- (500-mile-) wide Gulf of Mexico, and many warblers (warbler) fly from the American coast to Bermuda, a journey of about the same distance. For further information, see migration.

The bipedal gait of birds, dictated by modification of the forelimbs for flight, necessitates manipulation of food by the bill and feet. This poses problems in balance. The relative lengths of the segments of the legs must be such that, as the bird shifts from a standing to a sitting position, its centre of gravity remains over the feet. As some birds moved out of the trees and became terrestrial or aquatic, their legs were accordingly modified. In very large, slow-moving birds such as moas, the leg bones became very heavy. The toes became shorter, and the opposable first toe has been lost in rapidly running forms such as rheas (rhea) and ostriches. The ostrich is the fastest runner, crossing stretches of savanna at a speed of 72.5 km/hr (45 mph). The toes became very long in birds that walk on aquatic vegetation or soft ground. Jacanas (jacana) with their greatly elongated toes and claws walk over floating water weeds, and herons with long legs wade in shallow water. Wading birds developed long, thin legs, and climbing birds developed short legs with strongly curved, sharp claws. In swimming and diving birds, webs developed between the toes or lobes on the sides of the toes.

Terrestrial birds such as pheasants (pheasant) tend to walk; arboreal songbirds tend to hop as they travel from branch to branch. Tree dwellers such as woodpeckers (woodpecker), toucans (toucan), and the other members of order Piciformes (piciform), as well as parrots, can travel easily up and down trees because both of their outer toes face backward; in almost all other birds, only one toe faces to the rear. Parrots often walk along branches, and house sparrows (house sparrow) hop when they come to the ground, while palm warblers walk on the ground and some songbirds, such as American robins (robin) and European blackbirds, may both walk and hop. Some birds with small feet, such as swifts, hummingbirds, bee-eaters (bee-eater), and many hornbills (hornbill), use their feet only for perching and rarely walk at all. Other birds with robust feet, such as guinea fowl and rails, do most of their moving about on foot.

Some birds, such as the mallard, usually swim at the surface, feeding only as far underwater as they can reach by dipping their heads. Other ducks, such as scoters (scoter) and pochards (pochard), commonly dive to the bottom for their food, and cormorants (cormorant), auks, and loons pursue fish underwater. Sometimes loons are taken at remarkable depths in fishermen's nets and on set lines, indicating that they may dive as deep as 61 metres (200 feet). Emperor penguins, however, are the best divers, having been recorded at depths of 483 metres (1,584 feet).

Most birds build nests in which the eggs are laid. Nests vary widely: they may be a scrape in the sand, a deep burrow, a hole in a tree or rock, an open cup, a globular or retort-shaped mass with a side entrance tube, or an elaborately woven hanging structure. The materials with which nests are made also vary widely. Some nests are lined with small stones, others are built of dirt or mud with or without plant material. Sticks, leaves, algae, rootlets, and other plant fibres are used alone or in combination. Some birds seek out animal materials such as feathers, horsehair, or snakeskin. The nest materials may be held together by weaving, sewing, or felting the materials themselves or with mud or spider webs. Swifts (swift) use saliva to glue nest materials together and to attach the nest to the supporting structure. In at least one species of swift, the entire nest is made of saliva and is the prized ingredient of birds' nest soup in the Orient.

All birds incubate (incubation) their eggs, except megapodes (megapode) (mound builders), which depend on the heat generated by decaying vegetation or other external sources, and brood parasites such as cuckoos (cuckoo) and cowbirds (cowbird), which lay their eggs in the nests of other species. Murres (murre) and the king and emperor penguins (penguin) build no nest but incubate with the egg resting on top of the feet.

The earliest birds were probably insectivorous, as are many modern ones, and the latter have evolved many specializations for catching insects. Swifts, swallows, and nightjars (nightjar) have wide gapes for catching insects on the wing; some woodpeckers (woodpecker) can reach wood-boring grubs, whereas others can catch ants by probing anthills with their long, sticky tongues; thrashers (thrasher) dig in the ground with their bills; tree creepers (creeper) and woodhewers (woodcreeper) probe bark crevices; and warblers (warbler) glean insects from many kinds of vegetation. Raptorial birds (raptors (raptor) and owls) have evolved talons and hooked bills for feeding on larger animals, and vultures (vulture) have bare heads and tearing bills for feeding on carrion. Herons have spearlike bills and trigger mechanisms in the neck for catching fish, while kingfishers (kingfisher), terns, and boobies plunge into the water after similar prey. Long-billed waders probe for worms and other invertebrates. Of the many kinds of birds that feed on plant material, most use seeds, fruit, or nectar, which are high in food value; leaves and buds are eaten by fewer species. While some kinds of birds feed entirely on a single kind of food, others may take a wide range of foods, and many have seasonal changes in diet.

The structures associated with flight, even if they are vestigial or specialized for terrestrial or aquatic locomotion, easily distinguish birds from other animals. Whereas various skeletal and internal features are diagnostic of birds, feathers are unique to and present on all birds. Also unique to birds is their sound-producing organ, the syrinx. This avian analog to the voice box ( larynx) is most highly developed in the songbirds. The syrinx is located where the trachea (windpipe) divides into the bronchial tubes. Sound is produced by airflow that vibrates membranes formed from part of the trachea, bronchi, or both.

Like the scales of reptiles, and those on the feet of birds, feathers are made of keratin, a fibrous protein also found in hair. Feathers vary considerably in structure and function. Contour feathers form most of the surface of the bird, streamlining it for flight and often waterproofing it. The basal portion may be downy and thus act as insulation. The major contour feathers of the wing (remiges) and tail (rectrices) and their coverts function in flight. Contour feathers grow in tracts (pterylae) separated by bare areas (apteria) and develop from follicles in the skin.

Colour (coloration) in birds is caused by pigments and structure. Buffs, red browns, dark browns, and blacks are caused by melanins (melanin), pigments synthesized by the bird and laid down in granules. Yellows, oranges, and reds come from carotenoid or lipochrome pigments; these originate at least in part from the food and are diffused in the skin and feathers. porphyrin feather pigments occur in birds but less frequently than melanins and carotenoids. Blue colours in feathers are structural, based on a thin, porous layer of keratin overlying melanin pigment. Most greens result from the addition of yellow pigment to the structural blue colour. Iridescent (iridescence) colours result from the thinly laminated structure of the barbules and are enhanced by underlying melanin deposits.

The avian skeleton is notable for its strength and lightness, achieved by fusion of elements and by pneumatization (i.e., presence of air cavities). The skull represents an advance over that of reptiles in the relatively larger cranium with fusion of elements, made possible by the fact that birds have a fixed adult size. Birds differ from mammals in being able to move the upper mandible rather than the lower, relative to the cranium. When the mouth is opened, both the lower and upper jaws (jaw) move: the former by a simple, hingelike articulation with the quadrate bone at the base of the jaw, the latter through flexibility provided by a hinge between the frontal and nasal bones. As the lower jaw moves downward, the quadrate rocks forward on its articulation with the cranium, transferring this motion through the bones of the palate and the bony bar below the eye to the maxilla, the main bone of the upper jaw.

The sternum consists of a plate lying ventral to the thoracic cavity and a median keel extending ventrally from it. The plate and keel form the major area of attachment for the flight muscles. The bones of the pectoral girdle consist of the wishbone (furcula) and the paired coracoids and shoulder blades (scapulae). The sword-shaped scapula articulates with the coracoid and upper “armbone” ( humerus) and lies just dorsal to the rib basket. The coracoid articulates with the forward edge of the sternum and with the scapula, humerus, and furcula. The furcula connects the shoulder joints with the anterior edge of the keel of the sternum. It consists of paired collarbones (clavicles) and, probably, the median, unpaired interclavicle.

The bones of the forelimb (wing) are modified for flight with feathers. Major modifications include restriction of the motion of the elbow and wrist joints to one plane, reduction of the number of digits, loss of functional claws, fusion of certain bones of the “hand” (the metacarpals and most of the carpals) into a carpometacarpus, and modification of the elements, especially those toward the tip of the limb (distal), for the attachment of feathers. The wing bones are hollow, and the cavity in the humerus is connected with the air-sac system. As a general rule, large flying birds have proportionally greater pneumaticity in the skeleton than small ones. The highly pneumatic bones of large flying birds are reinforced with bony struts at points of stress. The humerus, radius, and ulna are well developed. The secondary flight feathers are attached to the ulna, which thus directly transmits force from the flight muscles to these feathers and is therefore relatively heavier than the radius. Two small wrist bones are present: the radiale, or scapholunar, and the ulnare (ulna), or cuneiform. The former lies between the distal end of the radius and the proximal part (the part toward the body) of the carpometacarpus. When the elbow joint is flexed (bent), the radius slides forward on the ulna and pushes the radiale against the carpometacarpus, which in turn flexes the wrist. Thus the two joints operate simultaneously. The U-shaped ulnare articulates with the ulna and the carpometacarpus. Anatomists differ on which bones of the reptilian “hand” are represented in the bird's wing. Embryological evidence suggests that the digits are II, III, and IV, but it is possible that they are actually I, II, and III. The carpometacarpus consists of fused carpals (bones of the wrist) and metacarpals (bones of the palm), metacarpals II and III (or III and IV) contributing the greater part of the bone. The bones of the “fingers” (phalanges) are reduced to one each on the outer and inner digits and two on the middle one. The primary flight feathers are attached to the carpometacarpus and digits, the number attached to each being characteristic of the various major groups of birds.

The avian lung differs from the type found in other land vertebrates in that several pairs of nonvascular air sacs are connected with the lungs. These extend into the pneumatic parts of the skeleton. Muscles between the sternum and trachea or along the trachea and bronchi vary tension on the membranes.

The testes (testis) of the male bird are internal, like those of reptiles. Intromittent organs are found in only a few groups (waterfowl, curassows (curassow), tinamous (tinamou), ratites). The distal part of the vas deferens (ductus deferens) (the seminal sac) becomes enlarged and convoluted in the breeding season and takes on both secretory and storage functions. In songbirds this enlargement and the adjacent part of the cloaca form a cloacal protuberance, a swelling visible on the outside of the bird. Usually only the left ovary and oviduct are functional. As the egg moves down the oviduct, the albumen, membranes, and shell are laid down. The gonads and accessory sexual organs of both sexes enlarge and regress seasonally. In the breeding season, the testes of finches may increase more than 300 times their winter size.

The origin of birds, feathers, and avian flight have long been hotly debated. That birds evolved from reptilian ancestors is universally accepted, but did they evolve from thecodont (thecodontian) reptiles or theropod dinosaurs? How and when did feathers evolve from the scaly reptilian integument? Did avian flight evolve from a terrestrial ancestor or an arboreal one? Such debates continue to be fueled by regular discoveries in China and Mongolia of remarkably preserved fossils from a critical period in avian evolution—the Early Cretaceous (Cretaceous Period) (144 million to 99 million years ago). The diversity of theropod dinosaurs, some with feathers, has greatly expanded our perspective of the evolution and early diversification of birds. Two major groups of early birds diversified dramatically in the Cretaceous Period: the Enantiornithines, an extinct subclass of birds; and Odontornithes, a primitive offshoot of the subclass Ornithurae that gave rise to modern birds at the end of the Cretaceous. Paleontologists now position Archaeopteryx, once thought to be a direct ancestor of birds, and several other fossil taxa at the base of the extinct enantiornithine lineage.

Feathers (feather) are complex and novel evolutionary structures. They did not evolve directly from reptilian scales, as once was thought. Current hypotheses propose that they evolved through an invagination of the epidermis around the base of a dermal papilla, followed by increasing complexity of form and function. They evolved before birds and even before avian flight. Thus, early feathers functioned in thermal insulation, communication, or water repellency, but not in aerodynamics and flight. Among extinct life forms, feathers are no longer considered a unique and diagnostic characteristic of birds. Feathers with modern features were present in a variety of forms on a variety of theropod dinosaurs. At least nine Cretaceous dinosaurs had featherlike structures. The details of some are questionable, but some, such as those of Sinornithosaurus and other basal dromaeosaurs, bear a resemblance to modern pennaceous feathers. Feathered dinosaurs did not survive the end of the Cretaceous Period, but birds did, and then they flourished.

The earliest undisputed fossil bird is Archaeopteryx lithographica (Archaeopteryx). About the size of a magpie, it resembled some reptiles and differed from modern birds in many ways, notably: (1) the jaws contained teeth set in sockets, (2) the sternum was short and not keeled, (3) the bones were not pneumatic, (4) the first two metacarpals were free, resulting in three movable digits of the “hand,” all with functional claws. Avian characteristics of Archaeopteryx included the possession of feathers and other skeletal features indicating that the creature represented an intermediate stage between reptiles and modern birds. The absence of a keel on the short sternum indicates that Archaeopteryx did not fly but glided. An opposable hallux, indicative of a perching type of foot, and clawed digits on the hand point to an arboreal existence.

The other major group of toothed Cretaceous birds, the Odonotornithes, included one of the best-known groups of fossil birds, Hesperornis and its relatives. These birds were highly specialized foot-propelled divers of the Late Cretaceous. Hesperornis was up to 1.8 metres (6 feet) long and had completely lost the power of flight. The sternum lacked a keel, the humerus was small and weak, and other elements of the wing were missing entirely. The pelvis and hindlimb had a strong but superficial resemblance to those of modern loons (loon) and grebes (grebe). However, two major features and several less obvious ones indicate that the resemblance was the result of convergent evolution rather than common ancestry. Hesperornis was remarkable for three features. It had (1) teeth set in grooves, not sockets, (2) a stout fourth toe with a unique rotary ball-and-flange type of articulation, and (3) tail (caudal) vertebrae with limited vertical motion, making the tail somewhat beaverlike in its action. Baptornis, a contemporary relative of Hesperornis, was smaller and less strongly modified. Though flightless, its wings were less reduced than Hesperornis, and it lacked the peculiar modifications of the fourth toe and caudal vertebrae.

By Pliocene (Pliocene Epoch) times (5.3 million to 1.8 million years ago), most modern genera were probably in existence. After the extinction of the dinosaurs and before large carnivorous mammals evolved, two groups of large flightless birds evolved to fill a similar niche. From the Late Paleocene to the Middle Eocene epochs, Diatryma and its relatives were major predators in the Northern Hemisphere. The largest species stood over 2.25 metres (7 feet) tall and had stout hooked beaks. They are of uncertain relationships but may have been distantly related to the cranes (crane) and rails (rail) (order Gruiformes (gruiform)). The second group, that of Phororhacos and related genera, had a long history (from the lower Oligocene to the middle Pliocene) in South America, which was without large carnivores until relatively recent times. The Phororhacos line evidently evolved from seriema-like stock and radiated into numerous genera and species, the largest of them (Onactornis) standing 2.5 metres (8 feet) tall and having a skull 80 cm (31 inches) long and 40 cm (16 inches) high.

10,100 living species of vertebrate (backboned) animals primarily adapted for flight with feathers. Warm-blooded with a 4-chambered heart; left systemic arch lost. Lower jaw articulates with cranium via the quadrate; teeth absent in living forms. Reproduction by hard-shelled eggs, nearly always incubated by one or both parents.

David Attenborough, The Life of Birds (1998), published in conjunction with a five-part video documentary series of the same name, is one of the best overviews of avian biology available. BirdLife International, Threatened Birds of the World (2000), encyclopaedically summarizes conservation status, includes maps and drawings. Michael Hutchins (ed.), Grzimek's Animal Life Encyclopedia, Vols. 8–11 (2003), includes sections on behaviour in the discussion of each species and group described. David Allen Sibley, The Sibley Guide to Bird Life & Behavior (2001), further describes the activities of birds. Josep del Hoyo, Andrew Elliott, and Jordi Sargatal (eds.), Handbook of Birds of the World (1992– ), still in progress, is a massive multivolume set with excellent paintings, maps, and photographs. Frank S. Todd, 10,001 Titillating Tidbits of Avian Trivia (1994), is a treasure trove of facts useful for conversations with fellow ornithophiles. Michael Graham Wells, World Bird Species Checklist: With Alternative English and Scientific Names (1998), gathers as a complete compendium the range of different names that have been used for each bird species. James F. Clements, Birds of the World: A Checklist (2000), lists all species and is intended for personal record keeping by birdwatchers.

Primarily continental in scope, some of the following are not generally considered field guides. Alan Poole and Frank Gill, The Birds of North America: Life Histories for the 21st Century (1992–2002), summarizes the biology and distributions of 720 species that nest regularly in North America, including Hawaii; David Sibley, National Audubon Society Sibley Guide to Birds (2000), serves as an advanced field guide to birds of North America. Stanley Cramp (ed.), Handbook of the Birds of Europe, the Middle East, and North Africa: The Birds of the Western Palearctic, 8 vol. (1977–94), is also available in disc and compact book format; Leslie H. Brown et al., The Birds of Africa (1982– ), continues to grow as the definitive reference for the continent's avifauna; Richard Grimmett, A Guide to the Birds of India (1999), functions as a fully illustrated, modern field guide; Stephen Marchant and Peter Higgins, Handbook of Australian, New Zealand, and Antarctic Birds (1990– ); Robert Ridgely and Guy Tudor, Birds of South America (1994), includes only the perching (passerine) birds, but many country-specific books are available that include all the bird orders. Craig Robson (preface), A Field Guide to the Birds of South-East Asia (2000), portrays mainland species; whereas John Ramsay MacKinnon, A Field Guide to the Birds of Borneo, Sumatra, Java, and Bali (1993), is complementary in that it covers the major islands of Indonesia. Peter Harrison, Seabirds: An Identification Guide (1985); and Lars Löfgren, Ocean Birds (1987), compile accounts of species not associated with land regions.

Aspects of bird locomotion and behaviour are covered in the benchmark work Georg Rüppell, Bird Flight (1977; originally published in German, 1975). A comprehensive explanation of avian activity is Robert Burton, Bird Behavior (1985). A commanding synthesis of current knowledge of the development, function, and evolution of bird songs and calls is provided by Donald E. Kroodsma, Edward H. Miller, and Henri Ouellet (eds.), Acoustic Communication in Birds (2nd ed., 1997). Another specialized aspect of behaviour is addressed by Chris Mead, Bird Migration (1983); and an overview of the evolution and diversity of bird nests and nest building is put forth in Nicholas E. Collias and Elsie C. Collias, Nest Building and Bird Behavior (1984). An excellent treatment of our rapidly changing perspectives on the diversity and function of social systems of birds is elucidated in J. David Ligon, The Evolution of Avian Breeding Systems (1999). A classic summary of the parental behaviour of birds is presented in Alexander F. Skutch, Parent Birds and Their Young (1976).

Works on the anatomy and physiology of birds include Frank B. Gill, Ornithology (1994); and Noble S. Proctor and Patrick J. Lynch, Manual of Ornithology: Avian Structure and Function (1993), the standard college reference for functional anatomy of birds, includes a compact disc. Classic studies of ornithological knowledge are Donald S. Farner, James R. King, and K.C. Parkes (eds.), Avian Biology, 8 vol. (1971–85); and A.S. King and J. McLelland, Form and Function in Birds, 4 vol. (1979–89). Theories of egg formation, embryo development within the egg, and post-hatching biology of birds are compared in J.M. Starck and Robert Ricklefs, Avian Growth and Development (1997).

The evolutionary history of birds with emphasis on the fossil record of both birds and their reptile ancestors is constructed by Alan Feduccia, The Origin and Evolution of Birds (1996). Paleontological relevance to current ornithology is put into focus by R.O. Prum, “Why Ornithologists Should Care About the Theropod Origin of Birds,” Auk, 19:1–17 (2002). A pioneering summary of the full taxonomic history of birds based on DNA analyses is explained in Charles G. Sibley and Burt L. Monroe, Phylogeny and Classification of Birds (1990).Frank Gill