Backboned animal of the class Aves, the biggest group of land vertebrates, characterized by warm blood, feathers, wings, breathing through lungs, and egg-laying by the female. Birds are bipedal; feet are usually adapted for perching and never have more than four toes. Hearing and eyesight are well developed, but the sense of smell is usually poor. No existing species of bird possesses teeth.

Most birds fly, but some groups (such as ostriches) are flightless, and others include flightless members. Many communicate by sounds (nearly half of all known species are songbirds) or by visual displays, in connection with which many species are brightly coloured, usually the males. Birds have highly developed patterns of instinctive behaviour. There are nearly 8,500 species of birds.

According to the Red List of endangered species published by the World Conservation Union (IUCN) for 1996, 11% of bird species are threatened with extinction.

Wing structure
The wing consists of the typical bones of a forelimb, the humerus, radius and ulna, carpus, metacarpus, and digits. The first digit is the pollex, or thumb, to which some feathers, known as ala spuria, or bastard wing, are attached; the second digit is the index, which bears the large feathers known as the primaries or manuals, usually ten in number. The primary feathers, with the secondaries or cubitals, which are attached to the ulna, form the large wing-quills, called remiges, which are used in flight.

Anatomy
The sternum, or breastbone, of birds is affected by their powers of flight: those birds which are able to fly have a keel projecting from the sternum and serving as the basis of attachment of the great pectoral muscles which move the wings. In birds that do not fly the keel is absent or greatly reduced. The vertebral column is completed in the tail region by a flat plate known as the pygostyle, which forms a support for the rectrices, or steering tailfeathers.

The legs are composed of the femur, tibia and fibula, and the bones of the foot; the feet usually have four toes, but in many cases there are only three. In swimming birds the legs are placed well back.

The uropygial gland on the pygostyle (bone in the tail) is an oil gland used by birds in preening their feathers, as their skin contains no sebaceous glands. The eyes have an upper and a lower eyelid and a semitransparent nictitating membrane with which the bird can cover its eyes at will.

The vascular system contains warm blood, which is kept usually at a higher temperature (about 41°C/106°F) than that of mammals; death from cold is rare unless the bird is starving or ill. The aortic arch (main blood vessel leaving the heart) is on the right side of a bird, whereas it is on the left in a mammal. The heart of a bird consists of a right and a left half with four chambers.

The lungs are small and prolonged into air-sacs connected to a number of air-spaces in the bones. These air-spaces are largest in powerful fliers, but they are not so highly developed in young, small, aquatic, and terrestrial birds. These air-spaces increase the efficiency of the respiratory system and reduce the weight of the bones. The lungs themselves are more efficient than those of mammals; the air is circulated through a system of fine capillary tubes, allowing continuous gas exchange to take place, whereas in mammals the air comes to rest in blind air sacs.

The organ of voice is not the larynx, but usually the syrinx, a peculiarity of this class formed at the bifurcation of the trachea (windpipe) and the modulations are effected by movements of the adjoining muscles.

Digestion
Digestion takes place in the oesophagus, stomach, and intestines in a manner basically similar to mammals. The tongue aids in feeding, and there is frequently a crop, a dilation of the oesophagus, where food is stored and softened. The stomach is small with little storage capacity and usually consists of the proventriculus, which secretes digestive juices, and the gizzard, which is tough and muscular and grinds the food, sometimes with the aid of grit and stones retained within it. Digestion is completed, and absorption occurs, in the intestine and the digestive caeca. The intestine ends in a cloaca through which both urine and faeces are excreted.

Nesting and eggs
Typically eggs are brooded in a nest and, on hatching, the young receive a period of parental care. The collection of nest material, nest building, and incubation may be carried out by the male, female, or both. The cuckoo neither builds a nest nor rears its own young, but places the eggs in the nest of another bird and leaves the foster parents to care for them.

The study of birds is called ornithology.

Conservation measures
In most countries today official protection is given to birds, especially those that counteract the spread of injurious insects, slugs, snails, mice, and voles, and are therefore useful to agriculture. Bird sanctuaries exist in most countries of Europe and in the USA, and the recent increase in their numbers reflects an upsurge of popular interest in birds. In Britain, the growth of the Royal Society for the Protection of Birds is evidence of this.

Territorial behaviour
Within individual species, males may defend an area or territory against competing males. The size of territory varies between species. The gannet, a seabird which nests in dense colonies on cliffs and rocky islets, may defend an area encompassing only the extent to which the sitting female can jab with her bill, whereas the robin defends a territory of half a hectare.

Once a songbird has selected its territory, it will sing to fulfil the dual purpose of advertising its presence to rival territory holders and attracting a female. The song must signify to the female that the singer is a territory-holding male of the correct species in breeding condition, and is therefore important as a species-isolating mechanism. Closely related species that overlap in some habitats, for example the chiffchaff and willow warbler, often have conspicuously different songs. A song must be sufficiently stereotyped to be recognizable to other members of the species, but there is still room for much variation within these confines. Thus individual chaffinches may possess several different songs within their repertoire, all recognizable chaffinch songs, but differing perhaps in the number of notes in the song or in the type and arrangement of these notes. In species that do not sing, visual display may perform the same functions as song.

Different bird species occupying the same area usually have different food requirements, different nesting habits, and specific song and courtship behaviour, which not only prevents interbreeding but also reduces competition between species.