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Temporal range:
Late Cretaceouspresent, 72–0 Ma[1][2] Possible Early Cretaceous or early Late Cretaceous origin based on molecular clock[3][4][5]
Red-crested turacoSteller's sea eagleRock doveSouthern cassowaryGentoo penguinBar-throated minlaShoebillGrey crowned craneAnna's hummingbirdRainbow lorikeetGrey heronEurasian eagle-owlWhite-tailed tropicbirdIndian peafowlAtlantic puffinAmerican flamingoBlue-footed boobyKeel-billed toucanBird Diversity 2013.png
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Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Sauropsida
Clade: Avemetatarsalia
Clade: Ornithurae
Class: Aves
Linnaeus, 1758[6]
Extant clades
  • Neornithes Gadow, 1883

Birds are a group of

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) 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.

Birds are

Middle to Late Cretaceous, and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 mya, which killed off the pterosaurs and all non-avian dinosaurs.[5]


by the parents. Most birds have an extended period of parental care after hatching.

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.

Evolution and classification

Archaeopteryx lithographica
is often considered the oldest known true bird.

The first

classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[7]


Aves and a sister group, the order

fossils, and assigning them, instead, to the broader group Avialae,[11] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.[citation needed

Gauthier and de Queiroz[12] identified four different definitions for the same biological name "Aves", which is a problem. 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, which corresponds to meaning number 4 below. He assigned other names to the other groups.[citation needed]






  Lizards and snakes

The birds' phylogenetic relationships to major living reptile groups
  1. Aves can mean all archosaurs closer to birds than to crocodiles (alternately Avemetatarsalia)
  2. Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
  3. Aves can mean those feathered dinosaurs that fly (alternately Avialae)
  4. Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "crown group", in this sense synonymous with Neornithes)

Under the fourth definition Archaeopteryx, traditionally considered one of the earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and

bird evolution, though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.[13]











Cladogram showing the results of a phylogenetic study by Cau, 2018.[14]

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]

Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition is synonymous to Avifilopluma.[19]

Dinosaurs and the origin of birds










Cladogram following the results of a phylogenetic study by Cau et al., 2015[20]
Simplified phylogenetic tree showing the relationship between modern birds and dinosaurs [21]

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. Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod feathered dinosaurs, contribute to this ambiguity.[24][25][26]

Anchiornis huxleyi is an important source of information on the early evolution of birds in the Late Jurassic period.[27]

The consensus view in contemporary

arboreal, have been able to glide, or both.[29][30] Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, recent studies suggest that the first avialans were omnivores.[31]


theory of evolution in the late 19th century. Archaeopteryx was the first fossil to display both clearly traditional reptilian characteristics—teeth, clawed fingers, and a long, lizard-like tail—as well as wings with flight feathers similar to those of modern birds. It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.[32]

Early evolution

Confuciusornis sanctus, a Cretaceous bird from China that lived 125 million years ago, is the oldest known bird to have a beak.[33]

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 oldest known paravian (and probably the earliest avialan) fossils come from the

Aurornis xui.[13]

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 manoeuvreing.[35]

Avialans diversified into a wide variety of forms during the

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]

A third stage of bird evolution starting with Ornithothoraces (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, keeled sternum and the alula, and the loss of grasping hands. [14]


















Cladogram following the results of a phylogenetic study by Cau et al., 2015[20]

Early diversity of bird ancestors



















Mesozoic bird phylogeny simplified after Wang et al., 2015's phylogenetic analysis[39]

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

perching adaptations and seem to have included shorebird-like species, waders, and swimming and diving species.[citation needed

The latter included the superficially

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).[41] Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers,[42] which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.[35]

A study on mosaic evolution in the avian skull found that the last common ancestor of all Neornithes might have had a beak similar to that of the modern hook-billed vanga and a skull similar to that of the Eurasian golden oriole. As both species are small aerial and canopy foraging omnivores, a similar ecological niche was inferred for this hypothetical ancestor.[43]

Diversification of modern birds





perching and other birds




Basal divergences of modern birds
based on
Sibley-Ahlquist taxonomy

All modern birds lie within the

taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800[47] to 10,758.[48]

The discovery of

Galloanserae, the earliest diverging lineage within Neognathae.[1]

Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the

molecular clocks, showed that while according to some studies, modern birds originated early in the Late Cretaceous in Western Gondwana, a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event. Modern birds expanded from West Gondwana to the Laurasia through two routes. One route was an Antarctic interchange in the Paleogene. This can be confirmed with the presence of multiple avian groups in Australia and New Zealand. The other route was probably through North America, via land bridges, during the Paleocene. This allowed the expansion and diversification of Neornithes into the Holarctic and Paleotropics.[55] On the other hand, the occurrence of Asteriornis in the Northern Hemisphere challenges biogeographical hypotheses of a Gondwanan origin of crown birds.[1]

Classification of bird orders

Cladogram of modern bird relationships based on Braun & Kimball (2021)[56]


Struthio camelus - Etosha 2014 (1) white background.jpg

Rhea white background.jpg

Apterygiformes (kiwis) Little spotted kiwi, Apteryx owenii, Auckland War Memorial Museum white background.jpg

Tinamiformes (tinamous) NothuraDarwiniiSmit white background.jpg

cassowaries) Emu RWD2 white background.jpg


Red Junglefowl by George Edward Lodge white background.png

Cuvier-97-Canard colvert.jpg


flamingos)Cuvier-87-Flamant rouge.jpg

Podicipediformes (grebes)Podiceps cristatus Naumann white background.jpg


Columbiformes (pigeons and doves) Meyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg

Mesitornithiformes (mesites)Monias benschi 1912 white background.jpg

Pterocliformes (sandgrouse)Pterocles quadricinctus white background.jpg


Otidiformes (bustards)Cayley Ardeotis australis flipped.jpg

Cuculiformes (cuckoos)British birds in their haunts (Cuculus canorus).jpg

Musophagiformes (turacos)Planches enluminées d'histoire naturelle (1765) (Tauraco persa).jpg

Cuvier-72-Grue cendrée.jpg

D'Orbigny-Mouette rieuse et Bec-en-ciseaux white background.jpg

Opisthocomiformes (hoatzin)Cuvier-59-Hoazin huppé.jpg


Caprimulgiformes (nightjars) Chordeiles acutipennis texensisAQBIP06CA.jpg


Nyctibiiformes (potoos) NyctibiusBracteatusSmit.jpg

Steatornithiformes (oilbird) Steatornis caripensis MHNT ZON STEA 1.jpg

Podargiformes (frogmouths) Batrachostomus septimus 01.jpg


Aegotheliformes (owlet-nightjars) Aegotheles savesi.jpg

White-eared Hummingbird (Basilinna leucotis) white background.jpg


Cuvier-95-Phaeton à bec rouge.jpg

Cuvier-72-Caurale soleil.jpg


Loon (PSF).png


Thalassarche chlororhynchos 1838.jpg

Sphenisciformes (penguins) Chinstrap Penguin white background.jpg

Ciconiiformes (storks) Weißstorch (Ciconia ciconia) white background.jpg

Cormorant in Strunjan, white background.png

Spot-billed pelican takeoff white background.jpg


Vintage Vulture Drawing white background.jpg

Golden Eagle Illustration white background.jpg

Strigiformes (owls)Cuvier-12-Hibou à huppe courte.jpg


Coliiformes (mousebirds) ColiusCastanonotusKeulemans.jpg


Leptosomiformes (cuckoo roller) Leptosomus discolor - 1825-1834 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (cropped).tif

Trogoniformes (trogons and quetzals)Harpactes fasciatus 1838 white background.jpg


A monograph of the Bucerotidæ, or family of the hornbills (Plate II) (white background).jpg


Cuvier-46-Martin-pêcheur d'Europe.jpg

Dendrocopos major -Durham, England -female-8 white background.jpg


Cariama cristata 1838 white background.jpg


NewZealandFalconBuller white background.jpg


Psittaciformes (parrots)Pyrrhura lucianii - Castelnau 2.jpg

Passeriformes (passerines)Cuvier-33-Moineau domestique.jpg

The classification of birds is a contentious issue.

Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the subject.[58] Most evidence seems to suggest the assignment of orders is accurate,[59] but scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.[52][60]


As of 2010, the genome had been sequenced for only two birds, the chicken and the zebra finch. As of 2022 the genomes of 542 species of birds had been completed. At least one genome has been sequenced from every order.[61][62] These include at least one species in about 90% of extant avian families (218 out of 236 families recognised by the Howard and Moore Checklist).[63]

Being able to sequence and compare whole genomes gives researchers many types of information, about genes, the DNA that regulates the genes, and their evolutionary history. This has led to reconsideration of some of the classifications that were based solely on the identification of protein-coding genes. Waterbirds such as

flamingos, for example, may have in common specific adaptations suited to their environment that were developed independently.[61][62]


small bird withpale belly and breast and patterned wing and head stands on concrete
The range of the house sparrow has expanded dramatically due to human activities.[64]

Birds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the snow petrel's breeding colonies up to 440 kilometres (270 mi) inland in Antarctica.[65] The highest bird diversity occurs in tropical regions. It was earlier thought that this high diversity was the result of higher speciation rates in the tropics; however recent studies found higher speciation rates in the high latitudes that were offset by greater extinction rates than in the tropics.[66] Many species migrate annually over great distances and across oceans; several families of birds have adapted to life both on the world's oceans and in them, and some seabird species come ashore only to breed,[67] while some penguins have been recorded diving up to 300 metres (980 ft) deep.[68]

Many bird species have established breeding populations in areas to which they have been

game bird.[69] Others have been accidental, such as the establishment of wild monk parakeets in several North American cities after their escape from captivity.[70] Some species, including cattle egret,[71] yellow-headed caracara[72] and galah,[73] have spread naturally far beyond their original ranges as agricultural expansion created alternative habitats although modern practices of intensive agriculture have negatively impacted farmland bird populations.[74]

Anatomy and physiology