Phylum: Chordata
Superclass: Tetrapoda
Clades: Eumaniraptora , Paraves Clade: Avialae


Birds (class Aves or clade Avialae) are feathered, winged, two-legged, warm-blooded, egg-laying vertebrates. 


An Australian Galah, Eolophus roseicapilla, also known as the Rose-breasted Cockatoo. image: R.Conan-Davies

Aves ranks as the tetrapod class with the most living species, approximately ten thousand. Extant birds belong to the subclass Neornithes, living worldwide and ranging in size from the 5 cm  bee hummingbird to the 2.75 m  ostrich. 

Dinosaur origins of birds

The fossil record indicates that birds emerged within the theropod dinosaurs during the Jurassic period, around 150 million years ago. Most researchers agree that modern-day birds are the only living members of the Dinosauria clade.

Modern birds

Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. 

Extant birds have wings; the most recent species without wings was the moa, which is generally considered to have become extinct in the 16th century. Wings are evolved forelimbs, and most bird species can fly. 

Outside features of a typical bird (morphology).  1. Beak 2. Head(often includes a crest of feathers) 3. Iris 4. Pupil 5. Mantle 6. Lesser coverts 7. Scapulars 8. Coverts 9. Tertials 10. Rump 11. Primaries 12. Vent 13. Thigh 14. Tibio-tarsal articulation 15. Tarsus 16. Feet 17. Tibia 18. Belly 19. Flanks 20. Breast 21. Throat 22. Wattle 23. Eyestripe. image: wikipedia

Flightless birds

Flightless birds include ratites, penguins, and diverse endemic island species. Some species of birds, particularly penguins and members of the Anatidae family, are adapted to swim. 

Birds are adapted to flight

Birds also have digestive and respiratory systems that are uniquely adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species; several bird species make and use tools, and many social species culturally transmit knowledge across generations.

Bird behaviours

Many species annually migrate great distances, and many more perform shorter irregular movements. 


The routes of satellite-tagged bar-tailed godwits migrating north from New Zealand. This species has the longest known non-stop migration of any species, up to 10,200 km. image: wikipedia

Birds are social, communicating with visual signals, calls, and 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, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have polygynous ("many females") or, rarely, polyandrous ("many males") breeding systems. Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.

Economic human value of birds

Many species are economically important, mostly as game or poultry. Some species, particularly songbirds and parrots, are popular as pets. Another use of birds is harvesting guano (droppings) for use as a fertiliser.

Human culture and birds 

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.

source adapted from


See also

Classification of birds


Birds are generally classified into two broad groups initially: Palaeognathae …

Anatomy of birds

A stylised dove skeleton. Key: 1. skull 2. cervical vertebrae 3. furcula 4. coracoid 5. uncinate processes of ribs 6. keel 7. patella 8. tarsometatarsus 9. digits 10. tibia (tibiotarsus) 11. fibula (tibiotarsus) 12. femur 13. ischium (innominate) 14. pubis (innominate) 15. illium (innominate) 16. caudal vertebrae 17. pygostyle 18. synsacrum 19. scapula 20. dorsal vertebrae 21. humerus 22. ulna 23. radius 24. carpus (carpometacarpus) 25. metacarpus (carpometacarpus) 26. digits 27. alula