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Birds are a group of warm-blooded vertebrates constituting the class Aves (/eviz/), 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. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich. There are about ten thousand living species, more than half of which are passerine, or "perching" birds. Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Wings, which are modified forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. The study of birds is called ornithology.

Many social species pass on knowledge across generations, which is considered a form of culture. Birds are social, communicating with visual signals, calls, and songs, and participating in such behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially (but not necessarily sexually) monogamous, usually for one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilised through sexual reproduction. They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.

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Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano (bird excrement) is harvested for use as a fertiliser. Birds figure throughout human culture. About 120 to 130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry.

The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[7]Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.[8] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the clade Theropoda.[9]

Aves and a sister group, the order Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.[10] However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents to the PhyloCode. Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils, and assigning them, instead, to the broader group Avialae,[11] on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives.[11]

Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem.[12] The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants,[12] which corresponds to meaning number 4 below. They assigned other names to the other groups.[12]

Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all theropods closer to birds than to Deinonychus",[15][16] with Troodon being sometimes added as a second external specifier in case it is closer to birds than to Deinonychus.[17] Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight, and the birds that descended from them.[12][18]

Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs[22] and, more specifically, members of Maniraptora, a group of theropods which includes dromaeosaurids and oviraptorosaurs, among others.[23] As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred. By the 2000s, discoveries in the Liaoning Province of northeast China, which demonstrated many small theropod feathered dinosaurs, contributed to this ambiguity.[24][25][26]

Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest bird-line archosaurs to the first maniraptoromorphs, i.e. the first dinosaurs closer to living birds than to Tyrannosaurus rex. The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase.[14][34] After the appearance of Maniraptoromorpha, the next 40 million years marked a continuous reduction of body size and the accumulation of neotenic (juvenile-like) characteristics. Hypercarnivory became increasingly less common while braincases enlarged and forelimbs became longer.[14] The integument evolved into complex, pennaceous feathers.[34]

The well-known probable early avialan, Archaeopteryx, dates from slightly later Jurassic rocks (about 155 million years old) from Germany. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering.[35]

Avialans diversified into a wide variety of forms during the Cretaceous period. Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves).[36] Increasingly stiff tails (especially the outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the pygostyle, an ossification of fused tail vertebrae.[14] In the late Cretaceous, about 100 million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size.[37] Around 95 million years ago, they evolved a better sense of smell.[38]

The first large, diverse lineage of short-tailed avialans to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. While they were the dominant group of avialans during the Cretaceous period, enantiornithes became extinct along with many other dinosaur groups at the end of the Mesozoic era.[36]

Many species of the second major avialan lineage to diversify, the Euornithes (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialised in eating fish and other small aquatic organisms. Unlike the Enantiornithes, which dominated land-based and arboreal habitats, most early euornithes lacked perching adaptations and likely included shorebird-like species, waders, and swimming and diving species.[40]

The latter included the superficially gull-like Ichthyornis[41] and the Hesperornithiformes, which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic.[36] The early euornithes also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithes retained teeth in other parts of the jaws).[42] Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers,[43] which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.[35]

The discovery of Vegavis from the Maastrichtian, the last stage of the Late Cretaceous proved that the diversification of modern birds started before the Cenozoic era.[48] The affinities of an earlier fossil, the possible galliform Austinornis lentus, dated to about 85 million years ago,[49] are still too controversial to provide a fossil evidence of modern bird diversification. In 2020, Asteriornis from the Maastrichtian was described, it appears to be a close relative of Galloanserae, the earliest diverging lineage within Neognathae.[1]

All modern birds lie within the crown group Aves (alternately Neornithes), which has two subdivisions: the Palaeognathae, which includes the flightless ratites (such as the ostriches) and the weak-flying tinamous, and the extremely diverse Neognathae, containing all other birds.[52] These two subdivisions have variously been given the rank of superorder,[53] cohort,[9] or infraclass.[54] Depending on the taxonomic viewpoint, the number of known living bird species is around 10,906[55][56] although other sources may differ in their precise number. e24fc04721