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Birds are a group of warm-blooded vertebrates constituting the class Aves ( / ˈeɪviːz / ), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.
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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.
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A Little Bird Told Me
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.
eBird transforms your bird sightings into science and conservation. Plan trips, find birds, track your lists, explore range maps and bird migration—all free.
Learn about birds, the vertebrate animals adapted for flight, from National Geographic. Explore photos, videos, and stories of different bird species, their anatomy, behavior, and evolution. Discover how birds are related to dinosaurs and how they adapt to various habitats and challenges.
Learn about the incredible diversity, value and importance of birds for the global ecosystem and human culture. Find out how BirdLife is working to protect birds and their habitats from extinction and climate change.
