Alternate titles: Aves, fowl
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bird, (class Aves), any of the more than 10,400 living species unique in having feathers, 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 and that they have 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 humans 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 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 in art. Aesop’s fables abound in bird characters. The Physiologus and its descendants, the bestiaries 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 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 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, which shares its name with the currency and is 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 hind limbs for perching. This basic plan has become so modified during the course of evolution that in some forms it is difficult to recognize.
Among flying birds, the wandering albatross has the greatest wingspan, up to 3.5 metres (11.5 feet), and the trumpeter swan perhaps the greatest weight, 17 kg (37 pounds). In the largest flying birds, part of the bone is replaced by air cavities (pneumatic skeletons) because the maximum size attainable by flying birds is limited by the fact that wing area varies as the square of linear proportions, and weight or volume as the cube. During the Pleistocene Epoch (2.6 million to 11,700 years ago) lived a bird called Teratornis incredibilis. Though similar to the condors of today, it had a larger estimated wingspan of about 5 metres (16.5 feet) and was by far the largest known flying bird.
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The smallest living bird is generally acknowledged to be the bee hummingbird of Cuba, which is 6.3 cm (2.5 inches) long and weighs less than 3 grams (about 0.1 ounce). The minimum size is probably governed by another aspect of the surface-volume ratio: the relative increase, with decreasing size, in surface through which heat can be lost. The small size of some hummingbirds may be facilitated by a decrease in heat loss resulting from their becoming torpid at night.
When birds lose the power of flight, the limit on their maximum size is increased, as can be seen in the ostrich and other ratites such as the emu, cassowary, and rhea. The ostrich is the largest living bird and may stand 2.75 metres (9 feet) tall and weigh 150 kg (330 pounds). Some recently extinct birds were even larger: the largest moas of New Zealand and the elephant birds of Madagascar may have reached over 3 metres (10 feet) in height.
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.
Importance to man
In addition to their importance in literature and legend, birds have been significant to human society in myriad ways. Birds and their eggs 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, or guillemots, and the young of some muttonbirds are even now harvested in large quantities. With the development of agrarian human cultures, several species of chickens, ducks, geese, and pigeons 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.
Besides being a food source, pigeons have long been bred and trained for carrying messages, their wartime use dating to the Roman era, according to Pliny the Elder. Messenger pigeons were widely used by German, British, and American forces in World Wars I and II and by the United States in the Korean War. In the South Seas, the ability of frigate birds to “home” to their nesting colonies enabled island inhabitants to send messages by these birds.
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, doves, 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 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, grebes, 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. Small finches and parrots 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 (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 (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 and blackbirds is also a problem, as is the nesting of albatrosses on airplane runways on Pacific islands. As a result of these problems, conferences on the control of avian pests are commonly held.
Although birds are subject to a great range of diseases and parasites, only a few of these are known to be capable of infecting man. Notable exceptions are ornithosis psittacosis, or parrot fever, a serious and sometimes fatal disease resembling viral pneumonia. The microorganism responsible for the disease is transmitted directly to man from pigeons, parrots, and a variety of other birds via their excrement. Encephalitis, an inflammation of the brain, is also serious, but this infection is transmitted to man and to his domestic animals via biting arthropods, including mosquitoes. West Nile virus can likewise be transmitted. Wild birds may also act as reservoirs for diseases that adversely affect domesticated birds.
The study of birds has contributed much to both the theoretical and practical aspects of biology. Charles Darwin’s studies of the Galapagos finches 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, Nikolaas Tinbergen, 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.
Man’s impact on bird populations is very strong. Since 1680 approximately 80 species of birds have become extinct, and a larger number are seriously endangered. While pollution and pesticides are important factors in the decline of certain large species, such as the peregrine falcon, osprey, and California condor, the destruction of natural areas and introduction of exotic animals and diseases have probably been the most devastating. Concerted efforts of research and conservation are required to ensure the survival of rare species.Robert W. Storer
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 cannot fly, instead spending much of their time in the water swimming with their paddlelike wings. Birds such as ostriches and emus have rudimentary wings but are permanently afoot. At the other extreme, long-winged swifts and frigate birds 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 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.Load Next Page