The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.^[8] Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.^[9] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the clade Theropoda.^[10]

Definition

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.^[11] 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,^[12] on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives.^[12]

Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem.^[13] 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,^[13] which corresponds to meaning number 4 below. They assigned other names to the other groups.^[13]

Reptiles

Squamates	Lizards & snakes

Pantestudines

 Turtles

Archosaurs

	Crocodiles

	Birds

The birds' phylogenetic relationships to major living reptile groups. (The turtles' position is uncertain: Some authorities embed them inside the Archosaurs, with birds and crocodiles.)

Aves can mean all archosaurs closer to birds than to crocodiles (alternately Avemetatarsalia)
Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
Aves can mean those feathered dinosaurs that fly (alternately Avialae)
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.^[14]

Maniraptoromorpha

	†Coelurus

†Ornitholestes

Maniraptoriformes

†Ornithomimosauria

Maniraptora

†Alvarezsauridae

Pennaraptora

	†Oviraptorosauria

	Paraves

Cladogram showing the results of a phylogenetic study by Cau, 2018.^[15]

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",^[16]^[17] with Troodon being sometimes added as a second external specifier in case it is closer to birds than to Deinonychus.^[18] 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.^[13]^[19]

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.^[20]

Dinosaurs and the origin of birds

Paraves

†Scansoriopterygidae

†Eosinopteryx

Eumaniraptora

	†Jinfengopteryx

	†Aurornis

	†Dromaeosauridae

	†Troodontidae

	Avialae

Cladogram following the results of a phylogenetic study by Cau et al., 2015^[21]

Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs^[23] and, more specifically, members of Maniraptora, a group of theropods which includes dromaeosaurids and oviraptorosaurs, among others.^[24] 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.^[25]^[26]^[27]

The consensus view in contemporary palaeontology is that the flying theropods, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids.^[29] Together, these form a group called Paraves. Some basal members of Deinonychosauria, such as Microraptor, have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both.^[30]^[31] Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, studies suggest that the first avialans were omnivores.^[32]

The Late Jurassic Archaeopteryx is well known as one of the first transitional fossils to be found, and it provided support for the 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.^[33]

Early evolution

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.^[15]^[35] 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.^[15] The integument evolved into complex, pennaceous feathers.^[35]

The oldest known paravian (and probably the earliest avialan) fossils come from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago. The avialan species from this time period include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui.^[14]

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.^[36]

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).^[37] 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.^[15] 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.^[38] Around 95 million years ago, they evolved a better sense of smell.^[39]

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. ^[15]

Avialae

†Anchiornis

†Archaeopteryx

†Xiaotingia

†Rahonavis

	†Jeholornis

	†Jixiangornis

Euavialae

†Balaur

Avebrevicauda

†Zhongjianornis

†Sapeornis

Pygostylia

†Confuciusornithiformes

	†Protopteryx

	†Pengornis

Ornithothoraces

Cladogram following the results of a phylogenetic study by Cau et al., 2015^[21]

Early diversity of bird ancestors

†Archaeorhynchus

†Patagopteryx

†Vorona

†Schizooura

†Hongshanornithidae

†Jianchangornis

†Songlingornithidae

†Gansus

†Apsaravis

Ornithurae

†Hesperornithes

†Ichthyornis

	†Vegavis

	Aves

Mesozoic bird phylogeny simplified after Wang et al., 2015's phylogenetic analysis^[40]

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.^[37]^[41]

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.^[42]

The latter included the superficially gull-like Ichthyornis^[43] 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.^[37] 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).^[44] Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers,^[45] which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.^[36]

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.^[46]

Diversification of modern birds

Aves

Palaeognathae

(ratites and tinamous)

Neognathae

Galloanserae	(landfowl and waterfowl)

Neoaves	(all other birds including perching birds)

Major groups of modern birds based on Sibley-Ahlquist taxonomy

Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the Early Cretaceous^[3]^[47] to the latest Cretaceous.^[48]^[4] Similarly, there is no agreement on whether most of the early diversification of modern birds occurred in the Cretaceous and associated with breakup of the supercontinent Gondwana or occurred later and potentially as a consequence of the Cretaceous–Palaeogene extinction event.^[49] This disagreement is in part caused by a divergence in the evidence; most molecular dating studies suggests a Cretaceous evolutionary radiation, while fossil evidence points to a Cenozoic radiation (the so-called 'rocks' versus 'clocks' controversy).

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.^[50] The affinities of an earlier fossil, the possible galliform Austinornis lentus, dated to about 85 million years ago,^[51] 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]

Attempts to reconcile molecular and fossil evidence using genomic-scale DNA data and comprehensive fossil information have not resolved the controversy.^[48]^[52] However, a 2015 estimate that used a new method for calibrating molecular clocks confirmed that while modern birds originated early in the Late Cretaceous, likely in Western Gondwana, a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event.^[7] Modern birds would have expanded from West Gondwana through two routes. One route was an Antarctic interchange in the Paleogene. The other route was probably via Paleocene land bridges between South America and North America, which allowed for the rapid expansion and diversification of Neornithes into the Holarctic and Paleotropics.^[7] On the other hand, the occurrence of Asteriornis in the Northern Hemisphere suggest that Neornithes dispersed out of East Gondwana before the Paleocene.^[1]

Classification of bird orders

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.^[53] These two subdivisions have variously been given the rank of superorder,^[54] cohort,^[10] or infraclass.^[55] The number of known living bird species is around 11,000^[56]^[57] although sources may differ in their precise numbers.

Cladogram of modern bird relationships based on Stiller et al. (2024).^[58]

Aves

Palaeognathae

Struthioniformes (ostriches)

	Tinamiformes (tinamous)

	Rheiformes (rheas)

	Apterygiformes (kiwis)

	Casuariiformes (emu and cassowaries)

Neognathae

Galloanserae

	Galliformes (chickens and relatives)

	Anseriformes (ducks and relatives)

Neoaves

Mirandornithes

	Phoenicopteriformes (flamingos)

	Podicipediformes (grebes)

Columbaves

Columbimorphae

Columbiformes (pigeons and doves)

	Mesitornithiformes (mesites)

	Pterocliformes (sandgrouse)

Otidimorphae

Cuculiformes (cuckoos)

	Otidiformes (bustards)

	Musophagiformes (turacos)

Elementaves

Opisthocomiformes (hoatzin)

Gruimorphae

	Gruiformes (rails and cranes)

	Charadriiformes (waders and relatives)

Strisores

Caprimulgiformes (nightjars)

Vanescaves

Nyctibiiformes (potoos)

Steatornithiformes (oilbird)

Podargiformes (frogmouths)

Daedalornithes

	Aegotheliformes (owlet-nightjars)

	Apodiformes (swifts, treeswifts and hummingbirds)

Phaethoquornithes

Eurypygimorphae

	Phaethontiformes (tropicbirds)

	Eurypygiformes (sunbittern and kagu)

Aequornithes

Gaviiformes^[59] (loons)

Austrodyptornithes

	Procellariiformes (albatrosses and petrels)

	Sphenisciformes (penguins)

Ciconiiformes (storks)

	Suliformes (boobies, cormorants, etc.)

	Pelecaniformes (pelicans, herons and ibises)

Telluraves

Afroaves

Strigiformes (owls)

Accipitrimorphae

	Cathartiformes (New World vultures)

	Accipitriformes (hawks and relatives)

Coraciimorphae

Coliiformes (mousebirds)

Cavitaves

Leptosomiformes (cuckoo roller)

Trogoniformes (trogons and quetzals)

Picocoraciae

Bucerotiformes (hornbills and relatives)

Picodynastornithes

	Coraciiformes (kingfishers and relatives)

	Piciformes (woodpeckers and relatives)

Australaves

Cariamiformes (seriemas)

Eufalconimorphae

Falconiformes (falcons)

Psittacopasserae

	Psittaciformes (parrots)

	Passeriformes (passerines)

The classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the subject.^[60] Most evidence seems to suggest the assignment of orders is accurate,^[61] but scientists disagree about the relationships among 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. Fossil and molecular evidence from the 2010s is providing an increasingly clear picture of the evolution of modern bird orders.^[48]^[52]

Genomics

As of 2010^[update], the genome had been sequenced for only two birds, the chicken and the zebra finch. As of 2022^[update] the genomes of 542 species of birds had been completed. At least one genome has been sequenced from every order.^[62]^[63] 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).^[64]

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 pelicans and flamingos, for example, may have in common specific adaptations suited to their environment that were developed independently.^[62]^[63]