Temporal range:
Early Cretaceous (Aptian) – Present,[1] 121–0 Mya
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
About this image
Examples of various avian orders.

Row 1: Red-crested turaco, shoebill, white-tailed tropicbird
Row 2: Steller's sea eagle, grey crowned crane, common peafowl
Row 3: Rock dove, Anna's hummingbird, Atlantic puffin
Row 4: Southern cassowary, rainbow lorikeet, American flamingo
Row 5: Gentoo penguin, grey heron, blue-footed booby
Row 6: Bar-throated minla, Eurasian eagle-owl, keel-billed toucan

Scientific classification e
Linnaeus, 1758[2]
Extant Orders and temporal range
See orders
* Infraclass Palaeognathae
  • Neornithes Gadow, 1883

Birds, also known as Aves or avian dinosaurs, are a group of endothermic vertebrates, 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 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They rank as the world's most numerically-successful class of tetrapods, with approximately ten thousand living species, more than half of these being passerines, sometimes known as perching birds. Birds have wings which are more or less developed depending on the species; the only known groups without wings are the extinct moa and elephant birds. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in flightless birds, including ratites, penguins, and diverse endemic island species of birds. 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 fossil record demonstrates that birds are modern feathered dinosaurs, having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs. The closest living relatives of birds are the crocodilians. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, retained primitive characteristics such as teeth and long bony tails. DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages. But birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving dinosaurs according to cladistics.

Some birds, especially corvids and parrots, are among the most intelligent animals; several bird species make and use tools, and many social species pass on knowledge across generations, which is considered a form of culture. Many species annually migrate great distances. Birds are social, communicating with visual signals, calls, and bird songs, and participating in such social behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous (referring to social living arrangement, distinct from genetic monogamy), usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have breeding systems that are polygynous (arrangement of one male with many females) or, rarely, polyandrous (arrangement of 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. Some birds, such as hens, lay eggs even when not fertilised, though unfertilised eggs do not produce offspring.

Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds (poultry and game) 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 prominently figure throughout human culture. About 120–130 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.[3] Recreational birdwatching is an important part of the ecotourism industry.

Evolution and classification

Slab of stone with fossil bones and feather impressions
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.[4] Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.[5] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.[6]


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.[7] However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents of the Phylocode system. 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 Avialae,[8] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

Gauthier[9] identified four different definitions for the same biological name "Aves", which is a problem. Gauthier 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.




Lizards (including 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 is an avialan, and not a member of Aves. Gauthier's 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.[10]

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."[11][12] 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.[9][13]

Dinosaurs and the origin of birds

Anchiornis huxleyi is an important source of information on the early evolution of birds in the Late Jurassic period.[14]









Cladogram following the results of a phylogenetic study by Cau et al., 2015.[15]

Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs,[16] and more specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.[17] 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.[18][19][20]

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.[21] Together, these form a group called Paraves. Some basal members of this group, 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.[22][23] Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, recent studies suggest that the first avialans were omnivores.[24]

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.[25]

Early evolution

White slab of rock left with cracks and impression of bird feathers and bone, including long paired tail feathers
Confuciusornis sanctus, a Cretaceous bird from China that lived 125 million years ago, is the oldest known bird to have a beak.[26]

















Cladogram following the results of a phylogenetic study by Cau et al., 2015.[15]

The earliest known 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.[10]

The well-known 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.[27]

Avialans diversified into a wide variety of forms during the Cretaceous Period.[28] 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). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors,[28] the tails of more advanced avialans were shortened with the advent of the pygostyle bone in the group Pygostylia. 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.[29] Around 95 million years ago, they evolved a better sense of smell.[30]

Early diversity of bird ancestors

Ichthyornis, which lived 93 million years ago, was the first known prehistoric bird relative preserved with teeth.


















Mesozoic bird phylogeny simplified after Wang et al., 2015's phylogenetic analysis.[31]

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.[28]

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 seem to have included shorebird-like species, waders, and swimming and diving species.

The latter included the superficially gull-like Ichthyornis[32] 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.[28] 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).[33] Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers,[34] which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.[27]

A study on mosaic evolution in the avian skull found that the last common ancestor of all neornithines 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.[35]

Diversification of modern birds





Other birds (Neoaves)




Basal divergences of modern birds
based on Sibley-Ahlquist taxonomy

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.[36] These two subdivisions are often given the rank of superorder,[37] although Livezey and Zusi assigned them "cohort" rank.[6] Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800[38] to 10,050.[39]

The discovery of Vegavis, a late Cretaceous member of the Anatidae, proved that the diversification of modern birds started before the Cenozoic.[40] The affinities of an earlier fossil, the possible galliform Austinornis lentus, dated to about 85 million years ago,[41] are still too controversial to provide a fossil evidence of modern bird diversification.

Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the Middle Cretaceous[1] to the latest Late Cretaceous.[42] Similarly, there is no agreement on whether most of the early diversification of modern birds occurred before or after the Cretaceous–Palaeogene extinction event.[43] This disagreement is in part caused by a divergence in the evidence; most molecular dating studies suggests a Cretaceous radiation, while fossil evidence points to a Cenozoic radiation (the so-called 'rocks' versus 'clocks' controversy). Previous attempts to reconcile molecular and fossil evidence have proved controversial,[43][44] but more recent estimates, using a more comprehensive sample of fossils and a new way of calibrating molecular clocks, showed that while modern birds originated early in the Late Cretaceous, a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event.[45]

Classification of bird orders

Cladogram of modern bird relationships based on Prum, R.O. et al. (2015)[42] with some clade names after Yuri, T. et al. (2013).[46]


Struthioniformes[47] (ostriches)Struthio camelus - Etosha 2014 (1) white background.jpg


Rheiformes (rheas)Rhea white background.jpg


Casuariiformes (cassowaries & emus)Casuario australiano white background.JPG

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

Aepyornithiformes (elephant birds)Elephant bird skeleton.png

Tinamiformes (tinamous)NothuraDarwiniiSmit white background.jpg

Dinornithiformes (moa)Giant moa skeleton.jpg


Galliformes (chickens and relatives) Red Junglefowl by George Edward Lodge white background.png

Anseriformes (ducks and relatives) Cuvier-97-Canard colvert.jpg


Caprimulgiformes[47] (nightjars)

Steatornithiformes (oilbird)

Nyctibiiformes (potoos)

Podargiformes (frogmouths)

Apodiformes (swifts and hummingbirds)Haaksnavelkolibrie.jpg


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

Otidiformes (bustards)Cayley Ardeotis australis flipped.jpg

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


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

Mesitornithiformes (mesites)Monias benschi 1912 white background.jpg

Pteroclidiformes (sandgrouse)Pterocles quadricinctus white background.jpg

Gruiformes (rails and cranes)Cuvier-72-Grue cendrée.jpg


Phoenicopteriformes (flamingos)Cuvier-87-Flamant rouge.jpg

Podicipediformes (grebes)Podiceps cristatus Naumann white background.jpg

Charadriiformes (waders and relatives)D'Orbigny-Mouette rieuse et Bec-en-ciseaux white background.jpg


Phaethontiformes (tropicbirds)Cuvier-95-Phaeton à bec rouge.jpg

Eurypygiformes (sunbittern and kagu)Cuvier-72-Caurale soleil.jpg


Gaviiformes[47] (loons)


Procellariiformes (albatrosses and petrels)

Sphenisciformes (penguins) Chinstrap Penguin white background.jpg

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

Suliformes (boobies, cormorants, etc.)

Pelecaniformes (pelicans, herons & ibises) Spot-billed pelican takeoff white background.jpg


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


Cathartiformes (New World vultures)Vintage Vulture Drawing white background.jpg

Accipitriformes (hawks and relatives)Golden Eagle Illustration white background.jpg


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


Coliiformes (mouse birds)


Leptosomiformes (cuckoo roller)


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


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

Coraciiformes (kingfishers and relatives)Cuvier-46-Martin-pêcheur d'Europe.jpg

Piciformes (woodpeckers and relatives)Atlante ornitologico (Tav. 26) (picchio verde).jpg


Cariamiformes (seriemas)Cariama cristata 1838 white background.jpg


Falconiformes (falcons)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. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds,[48] although it is frequently debated and constantly revised. Most evidence seems to suggest the assignment of orders is accurate,[49] 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.[50][42]

Other Languages
Acèh: Cicém
Адыгэбзэ: Къуалэбзухэр
адыгабзэ: Бзыу
Afrikaans: Voël
Alemannisch: Vögel
አማርኛ: ወፍ
Ænglisc: Fugol
العربية: طائر
aragonés: Aves
ܐܪܡܝܐ: ܛܝܪܐ
armãneashti: Puľiu
arpetan: Usél
অসমীয়া: চৰাই
asturianu: Páxaru
Avañe'ẽ: Guyra
авар: ХӀинчӀ
Aymar aru: Jamach'i
azərbaycanca: Quşlar
تۆرکجه: قوش
বাংলা: পাখি
Bahasa Banjar: Burung
Bân-lâm-gú: Chiáu
башҡортса: Ҡоштар
беларуская: Птушкі
беларуская (тарашкевіца)‎: Птушкі
भोजपुरी: चिरई
Bikol Central: Gamgam
Bislama: Pijin
български: Птици
Boarisch: Vegl
བོད་ཡིག: བྱ།
bosanski: Ptice
brezhoneg: Evn
буряад: Шубуун
català: Ocells
Cebuano: Langgam
čeština: Ptáci
chiShona: Shiri
chiTumbuka: Viyuni
Cymraeg: Aderyn
dansk: Fugle
Deitsch: Voggel
Deutsch: Vögel
Diné bizaad: Tsídii
dolnoserbski: Ptaški
eesti: Linnud
Ελληνικά: Πτηνά
эрзянь: Нармунть
español: Aves
Esperanto: Birdoj
estremeñu: Páxaru
euskara: Hegazti
فارسی: پرنده
Fiji Hindi: Chirriya
føroyskt: Fuglur
français: Oiseau
Frysk: Fûgels
Gaeilge: Éan
Gaelg: Ushag
Gàidhlig: Eun
galego: Aves
ГӀалгӀай: Оалхазараш
贛語: 雀仔
ગુજરાતી: પક્ષી
客家語/Hak-kâ-ngî: Tiâu
хальмг: Шовуд
Hausa: Tsuntsu
հայերեն: Թռչուններ
हिन्दी: पक्षी
hornjoserbsce: Ptaki
hrvatski: Ptice
Ido: Ucelo
Ilokano: Billit
Bahasa Indonesia: Burung
interlingua: Aves
Ирон: Мæргътæ
isiXhosa: Yentaka
íslenska: Fugl
italiano: Aves
עברית: עופות
Jawa: Manuk
Kabɩyɛ: Sumaɣ
ಕನ್ನಡ: ಪಕ್ಷಿ
Kapampangan: Ayup
ქართული: ფრინველები
kaszëbsczi: Ptôchë
қазақша: Құстар
kernowek: Edhen
Kiswahili: Ndege (mnyama)
Kreyòl ayisyen: Zwazo
kurdî: Balinde
Кыргызча: Куш
кырык мары: Кек
Ladino: Pasharos
лакку: Лелуххи
latgaļu: Putni
Latina: Aves
latviešu: Putni
Lëtzebuergesch: Vullen
лезги: НуькI
lietuvių: Paukščiai
Ligure: Aves
Limburgs: Veugel
lingála: Ndɛkɛ
Lingua Franca Nova: Avia
la .lojban.: lo cipni
magyar: Madarak
македонски: Птици
Malagasy: Vorona
മലയാളം: പക്ഷി
Malti: Għasfur
मराठी: पक्षी
მარგალური: მაფურინჯეეფი
مصرى: طير
Bahasa Melayu: Burung
Baso Minangkabau: Buruang
Mìng-dĕ̤ng-ngṳ̄: Cēu
мокшень: Нармонь
монгол: Шувуу
မြန်မာဘာသာ: ငှက်
Nāhuatl: Tototl
Na Vosa Vakaviti: Manumanu vuka
Nederlands: Vogels
Nedersaksies: Voegel
नेपाली: चरा
नेपाल भाषा: झंगः
日本語: 鳥類
Napulitano: Auciello
нохчийн: Олхазарш
Nordfriisk: Fögler
norsk: Fugler
norsk nynorsk: Fuglar
Nouormand: Ouaîsé
occitan: Ausèl
олык марий: Кайык
ଓଡ଼ିଆ: ପକ୍ଷୀ
oʻzbekcha/ўзбекча: Qushlar
ਪੰਜਾਬੀ: ਪੰਛੀ
پنجابی: پنچھی
پښتو: مرغه
Patois: Bod
Перем Коми: Кайез
ភាសាខ្មែរ: បក្សី
Picard: Oizo
Piemontèis: Osej
Plattdüütsch: Vagels
polski: Ptaki
português: Aves
română: Pasăre
rumantsch: Utschels
Runa Simi: Pisqu
русиньскый: Птахы
русский: Птицы
саха тыла: Көтөрдөр
ᱥᱟᱱᱛᱟᱲᱤ: ᱪᱮᱬᱮ
संस्कृतम्: पक्षिणः
sardu: Aves
Scots: Bird
Seeltersk: Fuugele
shqip: Zogjtë
sicilianu: Aceddu
සිංහල: කුරුල්ලෝ
Simple English: Bird
سنڌي: پکي
SiSwati: Tinyoni
slovenčina: Vtáky
slovenščina: Ptiči
ślůnski: Ptoki
Soomaaliga: Shimbir
کوردی: مەل
српски / srpski: Птице
srpskohrvatski / српскохрватски: Ptica
ၽႃႇသႃႇတႆး : ၼူၵ်ႉ
Basa Sunda: Manuk
suomi: Linnut
svenska: Fåglar
Tagalog: Ibon
தமிழ்: பறவை
Taqbaylit: Aylal
татарча/tatarça: Кошлар
తెలుగు: పక్షి
тоҷикӣ: Паранда
Tsetsêhestâhese: Ve'kese
ತುಳು: ಪಕ್ಕಿ
Türkçe: Kuş
удмурт: Тылобурдо
українська: Птахи
اردو: پرندہ
Vahcuengh: Duzroeg
vèneto: Oxei
vepsän kel’: Lindud
Tiếng Việt: Chim
Volapük: Böds
Võro: Tsirk
walon: Oujhea
West-Vlams: Veugel
Winaray: Tamsi
ייִדיש: פויגל
Yorùbá: Ẹyẹ
Zazaki: Mıriçıki
Zeêuws: Veugels
žemaitėška: Paukštē