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Although most birds are characterised by flight, the ratites were 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, 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. Some birds, such as the owls and nightjars are nocturnal. 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 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 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, whereas the young of hole-nesters are often totally incapable of unassited survival. "Fledging" is the process of a chick acquiring feathers until it can fly.
The exact boundary between dinosaurs 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.
Sibley & Monroe's Phylogeny and Classification of Birds (~1990) is a landmark work on the classification of birds (although frequently debated and constantly revised).
Birds are an important food source for humans. The most commonly eaten species is the domestic chicken and its eggs, although geese, pheasants and ducks are also widely eaten. Other birds that have been utilized for food include emus, ostriches, pigeons, grouse, quails, doves, songbirds and others.
Many species have become extinct through over hunting, such as the Passenger Pigeon.
Numerous species are commensal on man and have become pandemic: Common Pigeon or Rock Doves (Columba livia), House Sparrows (Passer domesticus), Starlings (Sturnus vulgaris). Homing pigeons were formerly used to carry messages, and the honeycreepers[?] lead some Africans to honey.
As birds are extra-sensitive to toxins, the Canary was often used in coalmines to indicate the presence of poisonous gases, so that the miners could escape.
Bird diseases that can be contracted by humans are: psittacosis, salmonellosis[?], campylobacteriosis[?], Newcastle's disease, mycobacteriosis (avian tuberculosis), influenza, giardiasis[?] and cryptosporiadiosis[?].