Added 28 November 2005

A Brief Overview of


Class Aves


Birds are, after the fish, the largest group of vertebrates in terms of species. They are familiar to practically every human being on earth, and for the most part evoke feelings of admiration and affection rather than the dislike often accorded to their reptilian forebears.

The link between birds and dinosaurs was noted as early as the nineteenth century when only a few dinosaur species had yet been unearthed and classified. Since then the link has been called into doubt, but of late seems to have been confirmed again by the discovery of dinosaurs in China that had a layer of feathers. The major discovery as regards the relationship between the two classes was however the famous Archaeopterx, which featured the feathers and wings of modern birds but retained some features of reptiles not found in today's feathered species. The table below (adapted from Jessop) sums up this mixture of reptilian and avian characteristics:

Reptilian only features Reptilian features retained in modern birds Avian traits not found in dinosaurs
Teeth Digitigrade bipedality, ie walking on the toes, usually with 3 toes pointing forward and 1 pointing backwards Clavicles fused medially as a furcula (better known as the wishbone)
Three free hand digits bearing talons Fused metatarsal bone Hip structure lacking pubic symphysis, with pubic bones paralleling the ischia
Long tail with 21 vertebrae Intertarsal joint  
Abdominal floating ribs Hind limbs rotated under trunk and oriented vertically  
  Three-digit hand (digits II, III and IV further reduced)  
  Articulation of the atlas with the skull by means of a single occipital condyle  
  Epidermal scales on the feet  

Jessop included feathers in the third column, but recent discoveries seem to indicate that some dinosaurs were growing feathers, possibly the evolutionary forebears of creatures such as Archaeopteryx.

The discovery of the close link between reptiles and birds led some authorities such as Robert Bakker to argue (perhaps jokingly?) that Class Aves should be abolished and the birds considered as part of the Dinosauria. However it seems that the modern avian characteristics of feathers and oviparity (egg-laying) shared by all birds make them unique enough to remain in their own class, Aves. Nevertheless this does raise the problem that class Reptilia becomes paraphyletic rather than monophyletic, ie it no longer includes all descendants of the most recent common ancestor. The argument both ways could continue for some time.

Incidentally, it should also be noted that despite their shared powers of flying, birds are only distantly related to either flying reptiles such as pterosaurs or flying mammals such as bats. Apart from obvious differences in body (and lack of scales or fur), the structure of the wing is rather different in the pterosaurs and bats.

Characteristics of modern birds

An adaptation found in birds of the passerine family (comprising over half of all birds) is the tendon passing over the heel and inserting on the terminal phalanges: this locks the toes round a perch such as a twig, even when the bird is asleep.


Since these are such an integral part of the avian class, it is worth examining them in a little detail. Feathers are grown from feather follicles through interaction of the epidermal epithelium with dermal papillae. They are made of keratin, the same substance that forms the scales of reptiles and nails and hair of humans and other mammals. Feathers may be raised by tiny dermal feather erector muscles (arrectores plumarum) on each follicle. There are several types of feathers, each with a distinctive function.

The senses

The avian eye is essentially reptilian, with a sclerotic ring of bones that ossify in the eyeball's outer coat (the sclera) and support the front of it. This may help reduce distortion of the eyeball in flight (see Jessop, 21.45). The focus is as in reptiles, but birds have an additional Crampton's muscle that bows the cornea despite the strong counterpressure of the sclerotic bones and a transparent third eyelid (the nicitating membrane) that can be extended from the nasal corner). The avian ear is similarly an adaptation of the reptilian but with a much higher number of sensory cells. Unlike reptiles, birds have poor senses of taste and smell.

Other organs

Birds maintain a metabolic body temperature that is typically in the lower 40s centigrade, high by mammalian standards. Similarly the avian heart is proportionately larger and beats more rapidly. It is four-chambered like the mammalian but differs in layout.

The avian respiratory system is very different to that of other tetrapods. The long trachea from the larynx to the base of the neck divides into two bronchi: the syrinx, the sound-producing organ, is located here. The bronchi lead in turn into the lungs. The lungs are attached to the body wall and hence change little in size during breathing. Instead of the usual vertebrate arrangement of being saccular with alveolar clusters at the ends of the branches, have parabronchi (air passages) running lengthwise through them. These open into thin-walled distensible air sacs at each end. During inhalation both anterior and posterior air sacs become inflated as the lungs become compressed: this and the expansion of the anterior air sacs pulls air forward through the lungs into the air sacs at both ends. As the air in the anterior air sacs is then expelled, this action pulls the air from the posterior air sacs through the lungs. Thus an essentially continuous cycle of air is flowing through the lungs. As the air circulates there is a crosscurrent exchange between air and blood capillaries. No residual air is left in the lungs after exhalation. This efficient system allows some birds to fly as high as about 20,000 ft (6,400m).

A significant difference between birds and other vertebrates is in the digestive system. Birds have a thin-walled crop above the stomach, which can expand to allow rapid eating and then slowly pass the food to the stomach. The stomach itself is made up of the proventriculus at the front and the muscular gizzard at the back. The gizzard may contain stones deliberately swallowed by the bird that then help to grind up seeds and similar food, since birds cannot chew - an adaptation also found in crocodiles and believed to have been a characteristic of the dinosaurs. Many birds also lack a gall bladder, the bile draining instead directly into the intestine via the bile duct. Three ducts from the pancreas lead into the intestine, and in the join between the intestine and the rectum there are two ceca housing symbiotic bacteria that help to digest fibrous food. The rectum itself is short but wide and opens into the cloaca, which receives opening of the urinary and reproductive ducts as in reptiles. The bird tongue for the most part does not greatly assist with eating, with some notable exceptions such as woodpeckers.

Avian kidneys are also rather reptilian, and many birds produce a similarly semisolid uric paste. Most birds lack a urinary bladder. Yet another reptilian link is found in the nasal salt glands which are also seen in some squamate and chelonian reptiles.

The avian brain differs from that of the other vertebrate classes in being especially solid. The cerebral hemispheres are roofed by a thin archipallium, with most of the grey mater deep within in the form of a rudimentary neopallium and large corpus striatum. Sensory analysis takes place in the thalamus, which relays messages to the striatum. The striatum organises the complex instinctive behaviour patterns found in most birds. Birds display an ability to learn, but many of their behavioural patterns appear to be genetically determined: they are also subject to imprinting, which takes place early in life but has lasting effects. In this combination of inbuilt behaviour patterns and ability to learn they seem to form a bridge between the reptiles and mammals. A prime example of this is migration, which instinct appears to be born with a bird but which is subject to other parameters such as the ability to see the night sky and the possibility of imprinting (see Jessop for examples), as well as learning to recognise resting places, landmarks and the like.


One striking feature of birds is that all, without exception, are egg layers, exceeding in this even the reptiles, of which some at least bear live young.

Male birds, with the exception of the ratites and ducks and geese, lack a penis and mate by the male either standing on his partner's back and pressing his cloaca against hers or the female everting her oviduct and inserting it into the male's cloaca. The female only has a single ovary, the left; if this is destroyed by disease, the undeveloped right gonad may develop into a testis, causing some females to act as males, eg among poultry. Avian eggs are proportionally much larger to the body than those of reptiles, and only one egg a day is laid while younger eggs pass down the oviduct. Reproduction is usually seasonal and often involves elaborate courtship rituals. Chicks may be either precocial (able to walk and feed themselves soon after hatching) or altricial (requiring feeding by adults): in the latter case the parents may be tireless in making foraging trips to feed their young.

Classification of modern birds

CLASS AVES         No. of species
  Subclass Archaeornithes reptile-like birds from late Jurassic and Cretaceous Archaeopteryx and kin  
  Subclass Neornithes all other birds, characterised by fused metacarpals, elongate digit III and 13 or fewer tail vertebrae.    
    Superorder Odontognathae (aka Paleognathae) Distinct type of palate    
      Ratites (with unkeeled sternum) and tinamous (with keeled sternum)   
      Casuariformes tall, unkeeled sternum, very reduced wings, male incubates eggs of one female Emus and cassowaries of Australia and New Guinea 4
      Dinornithiformes unkeeled sternum, wings extremely degenerate, contour feathers absent, 3- or 4-toed, male incubates eggs of one female Kiwis and extinct moas of New Zealand 3**
      Rheiformes large 3-toed flightless birds: keelless sternum, well-developed wings used in courtship dancing, plumulous remiges: females lay eggs in communal nest and the males performs the incubation South American rheas 2
      Struthioniformes tall, 2-toed, unkeeled sternum, plumed wings used in courtship dancing, females lay eggs in communal nests and share incubation duties with male African ostriches 1
      Tinamiformes keeled sternum, weak wings; ground-dwelling and grouse-like running birds of Central and South America 60*
    Superorder Neognathae mobile palate with some bones reduced    
      Keeled sternum: wings well-developed and used for flying in air and/or water:  
      Anseriformes 3 toes in web; spatulate beaks covered by soft epidermis with many tactile sensory pits except along horny edge Ducks and geese 161
      Apodiformes very short legs and tiny feet make walking impossible; wings stiff and pointed; capture insects in midair or feed on flower nectar while hovering Swifts and hummingbirds 400*
      Caprimulgiformes nocturnal catchers of insects in midair; wide, bristle-fringed mouths; Poorwills, nighthawks 95
      Charadriiformes 3 toes webbed fully or just at base; varying bill shapes; diet of fish or aquatic invertebrates Shorebirds such as sandpipers, avocets, stilts, oystercatchers, gulls, terns, woodcocks, lapwings, snipe, plovers, skimmers, auks, puffins 330
      Ciconiiformes feet and toes usually long, often used to wade; bills usually long, often used for stabbing fish Storks, herons and ibises, New World condors and vultures 90*
      Coliiformes small; 1st and 4th toes reversible; tail very long; mouselike scurrying along branches African mousebirds 6
      Columbiformes short legs, short necks, short slender bill; young fed on "pigeon's milk" from crop; worldwide distribution Pigeons 290*
      Coraciiformes beak usually very large in proportion to bird; toes 3 and 4 fused at base Kingfishers, hornbills, motmots, hoopoes, bee-eaters, rollers 200*
      Cuculiformes 2 toes forward, 2 toes behind with outer rear toes reversible; nest parasitism common among cuckoos Cuckoos, roadrunners 150
      Falconiformes hooked-beak diurnal predators that seize prey with talons Hawks, eagless, falcons, kites, Old World condors and vultures 270
      Galliformes usually ground-dwelling seed-eaters, often flocking; strong beaks and heavy feet Turkeys, pheasants, quail, grouse, chickens, megapodes 250
      Gaviiformes 3 toes webbed, feet set so far back as to make walking almost impossible Loons 4
      Gruiformes prairie and marsh breeders; worldwide distribution  Long-legged grassland-dwelling cranes, short-legged marsh-dwelling rails, lobe-toed aquatic coots 215
      Musophagiformes Medium- to large-sized forest dwellers; conspicuous patch of crimson on spread wing; brightly coloured beak; short and rounded wings; African Turacos 6
      Passeriformes mostly small size; often excellent singers; sociable; tendon mechanism locks toes around slender perches, including at sleep; insectivorous or seed-eaters Small terrestrial songbirds and perching birds, crow family, shrikes >5000
      Pelecaniformes web joins all 4 toes; gular throat pouch; fish-eaters Pelicans, cormorants, frigate birds, boobies 55*
      Piciformes 2 toes forward, 1 or 2 behind; nest in cavities; worldwide distribution Woodpeckers, honeyguides, toucans 380*
      Podicipediformes toes lobed, feet set far back on trunk, tail reduced to tuft of down; divers after small animals and plant material Grebes 18
      Procellariiformes tube-nosed oceanic birds; rhampotheca composed of several plates; tubular nostrils carry nasal salt gland exudates towards the bill tip; breed mainly on islands Albatrosses and petrels 100
      Psittaciformes bill hooked, upper jaw highly movable, fleshy tongue; foot with 2 toes forward, 2 rearward, prehensile capability including the grasping of food; mostly fruit-eaters; tropical distribution parrots 320
      Sphenisciformes marine birds with flipperlike wings to "fly" at speed through water; southern hemisphere Penguins 17-19
      Strigiformes nocturnal predators, large forward-set eyes ringed by radially set feathers; asymmetrical ears to assist sound location; toes feathered, plumage soft-edged for silent flight; powerful beaks owls 135
      Trogoniformes small, weak feet; brilliant plumage; short stout beak with bristles at base; long tail; pantropical distribution trogons, quetzal 35*

* Approximate number

** Not including extinct species

As with most taxonomy, this arrangement is always subject to revision, especially in the light of DNA analysis and fossil finds.


Integrated Principles of Zoology, Eleventh Edition, Cleveland P Hickman Jr, Larry S Roberts and Allan Larson, McGraw-Hill 2001.

Zoology: The Animal Kingdom. A Complete Course in 1000 Questions and Answers, Nancy M Jessop, McGraw-Hill 1988.


The Tree of Life Project has a useful page discussing the characteristics and conflicting theories on the classification of birds.

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