Aves - birds


Sarcopterygii; Tetrapoda; Amniota; Sauropsida; Reptilia; Diapsida
Archosauria; Dinosauria; Saurischia; Theropoda; Aves


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Show Lateral view of a Pigeon Skeleton Image
Lateral view of a Pigeon Skeleton
Show Pigeon Skeleton Image
Pigeon Skeleton
Show Posterio-dorsal view of a Pigeon Skeleton Image
Posterio-dorsal view of a Pigeon Skeleton
Show Close-up of the Wing Bones of a Pigeon Image
Close-up of the Wing Bones of a Pigeon

Birds evolved from a group of small theropod dinosaurs in the Middle-Late Jurassic.

Diversity and Lower Taxonomy:

The class Aves, containing around 10,000 extant species of bird, can be divided into two clades:

  • Palaeognathae: Includes the tinamous - 47 species in one family within a single order - and the ratites(flightless birds), containing 5 families - the ostriches, kiwis, rheas, emu and cassowaries. Although previously thought to be separate groups, it has recently been shown that the tinamous are actually embedded within the ratites, thus ratites as they have been traditionally considered are a paraphyletic group, as their most recent common ancestor is shared with the tinamous. The proposed phylogenetic tree for palaeognathans is shown below:
  • Neognathae: With nearly 10,000 species, this clade contains the remaining living modern birds. It splits early to form two taxa, the Galloanserae (ducks, fowl and their relatives), and the Neoaves.
  • The superorder Neoaves contains 26 orders, including many well known groups such as parrots, woodpeckers, penguins, flamingoes, owls, raptors and vultures, kingfishers, and the largest, most diverse and recognised group - the passerine, or perching, birds.
  • With around 5,300 species, the monophyletic order Passeriformes makes up over half of all bird species, including the most popular and studied of all birds, the songbirds (Oscines), which are characterised by a complex voicebox used in mate attraction, as well as territory defence. You probably hear these sounds outside your bedroom window every morning, as songbirds include many common species such as robins, blackbirds, magpies, finches and swallows.
  • The phylogenetic relationships of the Neoaves is a hotly debated topic, with many studies currently in progress. In 2008, Neoaves underwent a major upheaval and reshuffle, based on genetic studies from Hackett et al. (2008) and Ericson et al. (2006), placing many groups in dramatically different phylogenetic positions. For example, the birds of prey, traditionally grouped together in an order called Falconiformes, has been split into two separate orders - the new Falconiformes, including only the single family of true falcons, and the Accipitriformes, containing all other birds of prey, including hawks, eagles and vultures. Phylogenetically, the new Falconiformes have been relocated to become the sister clade of the parrots and the passerines, far away from the Accipitriformes (whose full evolutionary relationships remain somewhat cloudy).


  • Feathers, which are specialised structures modified from reptilian scales, for insulation, display, camouflage and flight.
  • The insulation from these feathers allowed birds (and perhaps their dinosaurian ancestors) to become endotherms - allowing them to raise their internal body temperature by retaining the heat energy generated as a by-product of metabolism - a feature they share with mammals.
  • Wings, formed of the humerus, radius, ulna, wrist and three digits (of these, the first and third are greatly reduced).
  • The first digit forms the alula, a specialised wing slit, which acts to reduce drag when flying by keeping air close to the wing.
  • Fused clavicles, forming the furcula (wishbone).
  • Large keeled sternum (breastbone), for the attachment of powerful flight muscles. This keel is greatly reduced in most flightless birds, such as ratites.
  • No teeth; replaced by a horny beak.
  • Caudal vertebrae are reduced and fused to form the pygostyle, which supports the tail feathers.
  • Sacral vertebrae fused to form a synsacrum, which connects with a broad, elongated pelvic girdle and ossified thoracic vertebrae. These features, combined with the pygostyle, form a rigid trunk - an important adaptation for flight stability.
  • Fused tibia and uppermost tarsals (ankle bones) to form the tibiotarsus - what we call the drumstick. The fibula is reduced and spike-like.
  • Tarsometatarsus, formed by the fusion of the outermost tarsals with the metatarsals (toe bones).
  • Reversed hallux (big toe) on feet, specialised for perching.
  • In most birds, many bones, such as the sternum, pectoral girdle and humeri, are hollow and air-filled, or pneumatic. These air spaces have two functions:
    1. They acts to balance out the overall body weight to keep it relatively light for flight, as the bones of the hind limbs are usually solid and heavy to position the centre of gravity on the legs for perching/standing.
    2. They aid in respiration, by acting as a space to contain air sacs involved in the one-way (as opposed to tidal) flow of air through a bird's faveolar lungs.

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