A | B | C | D | E | F | G | H | CH | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
| Birds Temporal range:
Possible Early Cretaceous or early Late Cretaceous origin based on molecular clock[3][4][5] | |
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Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sauropsida |
| Clade: | Archosauria |
| Clade: | Avemetatarsalia |
| Clade: | Dinosauria |
| Clade: | Theropoda |
| Clade: | Ornithurae |
| Class: | Aves Linnaeus, 1758[6] |
| Extant clades | |
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| Synonyms | |
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Birds are a group of warm-blooded vertebrates constituting the class Aves (/ˈeɪviːz/), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich. There are over 11,000 living species, more than half of which are passerine, or "perching" birds. Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Wings, which are modified forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. The study of birds is called ornithology.
Birds are feathered theropod dinosaurs and constitute the only known living dinosaurs. Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians. Birds are descendants of the primitive avialans (whose members include Archaeopteryx) which first appeared during the Late Jurassic. According to recent estimates, modern birds (Neornithes) evolved in the Late Cretaceous and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non-avian dinosaurs.[7]
Many social species pass on knowledge across generations, which is considered a form of culture. Birds are social, communicating with visual signals, calls, and songs, and participating in such behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially (but not necessarily sexually) monogamous, usually for one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilised 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.
Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being 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 fertiliser. Birds figure throughout human culture. About 120 to 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.
Evolution and classification
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The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[8] Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.[9] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the clade Theropoda.[10]
Definition
Aves and a sister group, the order Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.[11] However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents to the PhyloCode. Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils, and assigning them, instead, to the broader group Avialae,[12] on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives.[12]
Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem.[13] The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants,[13] which corresponds to meaning number 4 below. They assigned other names to the other groups.[13]
| The birds' phylogenetic relationships to major living reptile groups. (The turtles' position is uncertain: Some authorities embed them inside the Archosaurs, with birds and crocodiles.) |
- Aves can mean all archosaurs closer to birds than to crocodiles (alternately Avemetatarsalia)
- Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
- Aves can mean those feathered dinosaurs that fly (alternately Avialae)
- Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "crown group", in this sense synonymous with Neornithes)
Under the fourth definition Archaeopteryx, traditionally considered one of the earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and bird evolution, though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.[14]