Lemon-bellied Flycatcher
Many: see text

Birds are bipedal, warm-blooded, egg-laying vertebrates characterized primarily by feathers, forelimbs modified as wings, and hollow bones. There are almost 9000 known species of birds in the world.

Table of contents
1 Introduction
2 Reproduction
3 Evolution
4 Birds and humans
5 See also
6 Other meanings of the word "bird"


Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu.

Although most birds are characterised by flight, the ratites are flightless, and several other species, particularly on islands, lost this ability. Flightless birds include the penguins, Ostrich, kiwis, and the extinct Dodo. Flightless species are vulnerable to extinction when humans or the mammals they introduce arrive in their habitat, for example the Great Auk, flightless rails, and the moa of New Zealand.

Birds are a very differentiated class, with some feeding on nectar, seeds, insects, rodents, fish, carrion, or other birds. Most birds are diurnal, or active during the day. Some birds, such as the owls and nightjars are nocturnal or crepuscular (active during twilight hours). Many birds migrate long distances to utilize marginal habitats (e.g., Arctic Tern) while others spend almost all their time at sea (e.g. the Wandering Albatross).

Common characteristics of birds are the ability to fly using feathered wings, a bony beak with no teeth, the laying of hard-shelled eggs, high metabolic rate, and a light but strong skeleton. Birds are among the most extensively studied animal groups, with hundreds of academic journals devoted to their study.

To groom, or preen, their feathers, birds use beaks to brush away foreign particles.

The birds of a region are called the avifauna. This term is also the name of various bird parks, e.g., that in Alphen aan den Rijn.


Although most male birds have no external sex organs, the male does have two testes which become hundreds of times larger during the breeding season to produce sperm. The female's ovaries also become larger, although only the left ovary actually functions.

In the males of species without a phallus (see below), sperm is stored within the proctodeum compartment within the cloaca prior to copulation. During copulation, the female moves her tail to the side and the male either mounts the female from behind or moves very close to her. He moves the opening of his cloaca, or vent, close to hers, so that the sperm can enter the female's cloaca, in what is referred to as a cloacal kiss. This can happen very fast, sometimes in less than one second.

The sperm is stored in the female's cloaca for anywhere from a week to a year, depending on the species of bird. Then, one by one, eggs will descend from the female's ovaries and become fertilized by the male's sperm, before being subsequently laid by the female. The eggs will then continue their development in the nest.

Many waterfowl and some other birds, such as the ostrich and turkey, do possess a phallus. Except during copulation, it is hidden within the proctodeum compartment within the cloaca, just inside the vent. The avian phallus differs from the mammalian penis in several ways, most importantly in that it is purely a copulatory organ and is not used for dispelling urine.

After the eggs hatch, parent birds provide varying degrees of care in terms of food and protection. The chicks of ground-nesting birds such as larks and waders are often able to run virtually immediately after hatching; such birds are referred to as nidifugous. The young of hole-nesters, on the other hand, are often totally incapable of unassisted survival. "Fledging" is the process of a chick acquiring feathers until it can fly.


Birds are generally considered to have evolved from theropod dinosaurss.

The exact boundary between dinosaurss and birds is unclear, especially with the recent discoveries in North-east China (Liaoning Province) that support the theory that many small theropod dinosaurs had feathers. It should be noted that although ornithischian dinosaurs share the same hip structure as birds (bird-hipped), birds originated from the saurischian or lizard-hipped dinosaurs, and so arrived at this condition independently. In fact, it developed a third time among a peculiar group of theropods, the thurizinosauridae.

The early bird Archaeopteryx, from the Jurassic, is well-known as one of the first "missing links" to be found in support of evolution in the late 19th century, though it is probably not basal among the birds.

All modern birds are classified as Neornithes, with two other major groups, the Ichthyornithes and Hesperornithes having become extinct.

The flightless Palaeognaths, the Ostrich group, were first to diverge from the avian lineage, and it is now thought that the basal divergence from the remaining Neognathes was that of the Galloanseri, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (the pheasants, grouse and their allies. See the chart.


Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised).

Birds and humans

Birds are an important food source for humans. The most commonly eaten species is the domestic chicken and its eggs, although geese, pheasants, turkeyss and ducks are also widely eaten. Other birds that have been utilized for food include emus, ostriches, pigeons, grouse, quails, doves, woodcocks, songbirds and others, including small passerines such as finches..

At one time swans and flamingos were delicacies of the rich and powerful, although these are generally protected now.

Many species have become extinct through over-hunting, such as the Passenger Pigeon.

Numerous species are commensal on man and have become pandemic, for example Common Pigeon or Rock Doves (Columba livia), House Sparrows (Passer domesticus) and Starlings (Sturnus vulgaris). Other birds have been used by humans: for example Homing pigeons to carry messages (many are still kept for sport), falcons for hunting, cormorants for fishing, and honeycreepers to lead people to honey. Chickens and pigeons are popular subjects in experimental research in biology and comparative psychology. As birds are extra-sensitive to toxins, the Canary was often used in coal mines to indicate the presence of poisonous gases, so that the miners could escape.

Colorful, particularly tropical, birds (e.g., Parrotss, and Mynahs) are often kept as pets although this has lead to smuggling of some endangered species; CITES does considerable work to deter this.

Bird diseases that can be contracted by humans include: psittacosis, salmonellosis, campylobacteriosis, Newcastle's disease, mycobacteriosis (avian tuberculosis), influenza, giardiasis, and cryptosporiadiosis.

Chinstrap Penguin

See also

Regional articles: Bird families and taxonomic discussion are given in list of birds and Sibley-Ahlquist taxonomy.

Other meanings of the word "bird"

People called Bird

Places called Bird

"Bird" is slang for...

Like all slang terms, these may be confined to one or a few English-speaking countries.
  • The bird is a colloquial name for the one-fingered salute.
  • To give a performer "the bird" is to show rowdy contempt for his or her performance
  • A bird is a slang term for a woman, usually a young one, usually used in relation to sexual or relationship matters: "Me and my bird" = "My girlfriend and I". This is a reversal of the traditional usage: during the earlier part of the 20th century, a bird was always male. Hence "He's a strange old bird" meaning "he is a curious man".
  • "Doing bird" is spending time in prison