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Birds (class Aves) are a group of endothermic vertebrates, 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. Birds live worldwide and range in size from the {{safesubst:#invoke:convert|convert}} bee hummingbird to the {{safesubst:#invoke:convert|convert}} ostrich. They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds or, less accurately, as songbirds.

Scientific consensus is that birds are the last surviving lineage of dinosaurs, having evolved from feathered dinosaur ancestors within the theropod group of saurischian dinosaurs. The fossil record indicates that true birds first appeared during the Cretaceous period, around 100 million years ago.<ref name="brown2011">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> However, primitive bird-like "stem-birds" that lie outside class Aves proper, in the group Avialae, have been found dating back to the mid-Jurassic period.<ref name=divergence/> Many of these early stem-birds, such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks and long bony tails.<ref name=divergence/><ref name="Alonso">{{#invoke:Citation/CS1|citation |CitationClass=journal }} PDF fulltext Supplementary info</ref>

Birds have more or less developed wings; the only known groups without wings are the extinct moa and elephant birds. Bird wings, which evolved from forelimbs, enabled birds the ability of bird flight. The digestive and respiratory systems of birds are also uniquely adapted for flight, although further speciation has led to some flightless birds, including ratites, penguins, and diverse endemic island species of birds. Some bird species of aquatic environments, particularly the aforementioned flightless penguins, and also members of the duck family, have also evolved for swimming. Birds, specifically Darwin's finches, played an important part in the inception of Darwin's theory of evolution by natural selection.

Some birds, especially corvids and parrots, are among the most intelligent animals; several bird species make and use tools, and many social species pass on knowledge across generations, which is considered a form of culture. Many species annually migrate great distances. Birds are social, communicating with visual signals, calls, and bird 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. Birds produce offspring by laying eggs which are fertilized through sexual reproduction. They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as hens, lay eggs even when not fertilized, though unfertilized eggs do not produce offspring.

Many species of birds are economically important. Domesticated and undomesticated birds (poultry and game) are important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano (bird excrement) is harvested for use as a fertilizer. 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. Recreational birdwatching is an important part of the ecotourism industry.

Bird sections
Intro  Evolution and classification  Distribution  Anatomy and physiology  Behaviour  Ecology  Relationship with humans  See also  External links  

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