| Birds are bipedal, warm-blooded, | | | | Gansus and the Hesperornithiformes, a |
| oviparous vertebrate animals | | | | group of flightless divers resembling |
| characterized primarily by feathers, | | | | grebes and loons. |
| forelimbs modified as wings, and (in | | | | The recently discovered dromaeosaur |
| most) hollow bones. All birds reproduce | | | | Cryptovolans was capable of powered |
| sexually, although parthenogenetic eggs | | | | flight, possessed a sternal keel and had |
| are known to be produced by the | | | | ribs with uncinate processes. In fact, |
| domesticated turkey on occasion and | | | | Cryptovolans makes a better "bird" than |
| suspected to occur in its wild ancestor | | | | Archaeopteryx which is missing some of |
| . | | | | these modern bird features. Because of |
| Birds range in size from the tiny | | | | this, some paleontologists have |
| hummingbirds to the huge Ostrich and | | | | suggested that dromaeosaurs are actually |
| Emu. Depending on the taxonomic | | | | basal birds whose larger members are |
| viewpoint, there are about | | | | secondarily flightless, i.e. that |
| 8,800–10,200 living bird species (and | | | | dromaeosaurs evolved from birds and not |
| about 120–130 that have become extinct | | | | the other way around. Evidence for this |
| in the span of human history) in the | | | | theory is currently inconclusive, but |
| world, making them the most diverse | | | | digs continue to unearth fossils |
| class of terrestrial vertebrates. | | | | (especially in China) of the strange |
| Birds feed on nectar, plants, seeds, | | | | feathered dromaeosaurs. At any rate, it |
| insects, fish, mammals, carrion, or | | | | is fairly certain that avian flight |
| other birds. | | | | existed in the mid-Jurassic and was |
| Most birds are diurnal, or active during | | | | "tried out" in several lineages and |
| the day, but some birds, such as the | | | | variants by the mid-Cretaceous. |
| owls and nightjars, are nocturnal or | | | | Although ornithischian (bird-hipped) |
| crepuscular (active during twilight | | | | dinosaurs share the same hip structure |
| hours), and many coastal waders feed | | | | as birds, birds actually originated from |
| when the tides are appropriate, by day | | | | the saurischian (lizard-hipped) |
| or night. | | | | dinosaurs (if the dinosaurian origin |
| Many birds migrate long distances to | | | | theory is correct), and thus arrived at |
| utilise optimum habitats (e.g., Arctic | | | | their hip structure condition |
| Tern) while others spend almost all | | | | independently. In fact, the bird-like |
| their time at sea (e.g. the Wandering | | | | hip structure also developed a third |
| Albatross). Some, such as Common Swifts, | | | | time among a peculiar group of |
| stay aloft for days at a time, even | | | | theropods, the Therizinosauridae. |
| sleeping on the wing. | | | | An alternate theory to the dinosaurian |
| Common characteristics of birds include | | | | origin of birds, espoused by a few |
| a bony beak with no teeth, the laying of | | | | scientists (most notably Lary Martin and |
| hard-shelled eggs, high metabolic rate, | | | | Alan Feduccia), states that birds |
| a 4-chambered heart, and a light but | | | | (including maniraptoran "dinosaurs") |
| strong skeleton. Most birds are | | | | evolved from early archosaurs like |
| characterised by flight, though the | | | | Longisquama, a theory which is contested |
| ratites are flightless, and several | | | | by most other scientists in |
| other species, particularly on islands, | | | | paleontology, and by experts in feather |
| have also lost this ability. Flightless | | | | development and evolution such as R.O. |
| birds include the penguins, ostrich, | | | | Prum. See the Longisquama article for |
| kiwi, and the extinct Dodo. Flightless | | | | more on this alternative. |
| species are vulnerable to extinction | | | | Modern birds are classified in |
| when humans or the mammals they | | | | Neornithes, which are now known to have |
| introduce arrive in their habitat. The | | | | evolved into some basic lineages by the |
| Great Auk, flightless rails, and the moa | | | | end of the Cretaceous (see Vegavis). The |
| of New Zealand, for example, all became | | | | Neornithes are split into the |
| extinct due to human influence. | | | | Paleognathae and Neognathae. The |
| Birds are among the most extensively | | | | paleognaths include the tinamous (found |
| studied of all animal groups. Hundreds | | | | only in Central and South America) and |
| of academic journals and thousands of | | | | the ratites. The ratites are large |
| scientists are devoted to bird research, | | | | flightless birds, and include ostriches, |
| while amateur enthusiasts (called | | | | cassowaries, kiwis and emus (though some |
| birdwatchers, twitchers or, more | | | | scientists suspect that the ratites |
| commonly, birders) probably number in | | | | represent an artificial grouping of |
| the millions. | | | | birds which have independently lost the |
| There is significant evidence that birds | | | | ability to fly in a number of unrelated |
| evolved from theropod dinosaurs, | | | | lineages). The basal divergence from the |
| specifically, that birds are members of | | | | remaining Neognathes was that of the |
| Maniraptora, a group of theropods which | | | | Galloanseri, the superorder containing |
| includes dromaeosaurs and oviraptorids, | | | | the Anseriformes (ducks, geese and |
| among others. As more non-avian | | | | swans), and the Galliformes (the |
| theropods that are closely related to | | | | pheasants, grouse, and their allies). |
| birds are discovered, the formerly clear | | | | See the chart for more information. |
| distinction between non-birds and birds | | | | The classification of birds is a |
| becomes less so. Recent discoveries in | | | | contentious issue. Sibley & Ahlquist's |
| northeast China (Liaoning Province), | | | | Phylogeny and Classification of Birds |
| demonstrating that many small theropod | | | | (1990) is a landmark work on the |
| dinosaurs had feathers, contribute to | | | | classification of birds (although |
| this ambiguity. | | | | frequently debated and constantly |
| The basal bird Archaeopteryx, from the | | | | revised). A preponderance of evidence |
| Jurassic, is well-known as one of the | | | | seems to suggest that the modern bird |
| first "missing links" to be found in | | | | orders constitute accurate taxa. |
| support of evolution in the late 19th | | | | However, scientists are not in agreement |
| century, though it is not considered a | | | | as to the relationships between the |
| direct ancestor of modern birds. | | | | orders; evidence from modern bird |
| Confuciusornis is another early bird; it | | | | anatomy, fossils and DNA have all been |
| lived in the Early Cretaceous. Both may | | | | brought to bear on the problem but no |
| be predated by Protoavis texensis, | | | | strong consensus has emerged. More |
| though the fragmentary nature of this | | | | recently, new fossil and molecular |
| fossil leaves it open to considerable | | | | evidence is providing an increasingly |
| doubt if this was a bird ancestor. Other | | | | clear picture of the evolution of modern |
| Mesozoic birds include the | | | | bird orders. See also: Sibley-Ahlquist |
| Enantiornithes, Yanornis, Ichthyornis, | | | | taxonomy. |