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For other uses, see Bird (disambiguation).
"Aves" redirects here. For other uses, see Aves (disambiguation).
| Birds Fossil range: Late Jurassic – Recent | ||||||||||
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 Double-crested Cormorant, Phalacrocorax auritus | ||||||||||
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| Orders | ||||||||||
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About two dozen - see section below |
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Birds (class Aves) are bipedal, warm-blooded, vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.7 m (9 ft) Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 150–200 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 155–150 Ma.
Modern birds are 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. All birds have forelimbs modified as wings and most can fly, with some exceptions including ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.
Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviours including 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 breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.
Many species are of economic importance, mostly as sources of food acquired through hunting or farming. Some species, particularly songbirds and parrots, are popular as pets. Other uses include the harvesting of guano (droppings) for use as a fertiliser. Birds figure prominently in all aspects of human culture from religion to poetry to popular music. About 120–130 species have become extinct as a result of human activity since the 17th century, and hundreds more before then. Currently about 1,200 species of birds are threatened with extinction by human activities, though efforts are underway to protect them.
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Archaeopteryx, the earliest known bird
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.del Hoyo, Josep; Andy Elliott & Jordi Sargatal (1992). Handbook of Birds of the World, Volume 1: Ostrich to Ducks. Barcelona: Lynx Edicions. ISBN 84-87334-10-5. Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. (Latin) Linnaeus, Carolus (1758). Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. Holmiae. (Laurentii Salvii), 824. Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.Livezey, Bradley C.; Richard L. Zusi (January 2007). "Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion". Zoological Journal of the Linnean Society 149 (1): 1–95. doi:10.1111/j.1096-3642.2006.00293.x. Aves and a sister group, the clade Crocodilia, together are the sole living members of the reptile clade Archosauria. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica. Padian, Kevin (1997). "Bird Origins", in Philip J. Currie & Kevin Padian (eds.): Encyclopedia of Dinosaurs. San Diego: Academic Press, 41–96. ISBN 0-12-226810-5. Archaeopteryx, from the Kimmeridgian stage of the Late Jurassic (some 155–150 million years ago), is the earliest known bird under this definition. Others, including Jacques Gauthier and adherents of the Phylocode system, have defined Aves to include only the modern bird groups, excluding most groups known only from fossils, and assigning them, instead, to the AvialaeGauthier, Jacques (1986). "Saurischian Monophyly and the origin of birds", in Kevin Padian: The Origin of Birds and the Evolution of Flight, 1–55. ISBN 0-940228-14-9. in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
All modern birds lie within the subclass Neornithes, which has two subdivisions: the Paleognathae, containing mostly flightless birds like ostriches, and the wildly diverse Neognathae, containing all other birds. These two subdivisions are often given the rank of superorder,Bird Biogeography (retrieved 27 January 2008) although Livezey & Zusi assigned them "cohort" rank. Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800Clements, James F. (2007). The Clements Checklist of Birds of the World, 6th edition, Ithaca: Cornell University Press. ISBN 978-0-8014-4501-9. to 10,050.Gill, Frank (2006). Birds of the World: Recommended English Names. Princeton: Princeton University Press. ISBN 978-0-691-12827-6.
Confuciusornis, a Cretaceous bird from China
Fossil evidence and intensive biological analyses have demonstrated beyond any reasonable doubt that birds are theropod dinosaurs. More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.Paul, Gregory S. (2002). "Looking for the True Bird Ancestor", Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: John Hopkins University Press, 171–224. ISBN 0-8018-6763-0. As scientists discover more non-avian theropods that are closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate that many small theropod dinosaurs had feathers, contribute to this ambiguity.Norell, Mark; Mick Ellison (2005). Unearthing the Dragon: The Great Feathered Dinosaur Discovery. New York: Pi Press. ISBN 0-13-186266-9.
The consensus view in contemporary paleontology is that the birds, Aves, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Together, these three form a group called Paraves. The basal dromaeosaur Microraptor has features which may have enabled it to glide or fly. The most basal deinonychosaurs are very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, and/or may have been able to glide.Turner, Alan H.; Pol, Diego; Clarke, Julia A.; Erickson, Gregory M.; and Norell, Mark (2007). "A basal dromaeosaurid and size evolution preceding avian flight" (pdf). Science 317: 1378-1381. doi:10.1126/science.1144066. Xing, X., Zhou, Z., Wang, X., Kuang, X., Zhang, F., and Du, X. (2003). "Four-winged dinosaurs from China." Nature, 421: 335–340.
The Late Jurassic Archaeopteryx is well-known as one of the first transitional fossils to be found and it provided support for the theory of evolution in the late 19th century. Archaeopteryx has clearly reptilian characters: teeth, clawed fingers, and a long, lizard-like tail, but it has finely preserved wings with flight feathers identical to those of modern birds. It is not considered a direct ancestor of modern birds, but is the oldest and most primitive member of Aves or Avialae, and it is probably closely related to the real ancestor. It has even been suggested that that Archaeopteryx was a dinosaur that was no more closely related to birds than were other dinosaur groups, Thulborn, R.A. (1984) "The avian relationships of Archaeopteryx, and the origin of birds." Zoological Journal of the Linnean Society 82:119-158 and that Avimimus was more likely to be the ancestor of all birds than Archaeopteryx. Kurzanov, S.M. (1987) "Avimimidae and the problem of the origin of birds" "Transactions of the joint Soviet - Mongolian Paleontological Expedition 31:31-94
There have been many controversies in the study of the origin of birds. Early disagreements included whether birds evolved from dinosaurs or more primitive archosaurs. Within the dinosaur camp there were disagreements as to whether ornithischian or theropod dinosaurs were the more likely ancestors.Heilmann, Gerhard. "The origin of birds" (1927) "Dover Publications", New York. Although ornithischian (bird-hipped) dinosaurs share the hip structure of modern birds, birds are thought to have originated from the saurischian (lizard-hipped) dinosaurs, and therefore evolved their hip structure independently.Rasskin-Gutman, Diego; Angela D. Buscalioni (March 2001). "Theoretical morphology of the Archosaur (Reptilia: Diapsida) pelvic girdle". Paleobiology 27 (1): 59–78. doi:10.1666/0094-8373(2001)027<0059:TMOTAR>2.0.CO;2 In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae.
Scientists Larry Martin and Alan Feduccia believe that birds are not dinosaurs, but that birds evolved from early archosaurs like Longisquama. The majority of their publications argued that the similarities between birds and maniraptoran dinosaurs were convergent, and that the two were unrelated. In the late 1990\'s the evidence that birds were maniraptorans became almost indisputable, so Martin and Feduccia adopted a modified version of a hypothesis by dinosaur artist Gregory S. Paul; where maniraptorans are secondarily flightless birds but,Paul, Gregory S. "Were some Dinosaurs Also Neoflightless Birds?" in Dinosaurs of the Air, pp. 224–58. in their version, birds evolved directly from Longisquama. Thus birds are still not dinosaurs, but neither are most of the known species that are currently classified as theropod dinosaurs. Maniraptorans are, instead, flightless, archosaurian, birds.Feduccia, Alan; Theagarten Lingham-Soliar, J. Richard Hinchliffe (November 2005). "Do feathered dinosaurs exist? Testing the hypothesis on neontological and paleontological evidence". Journal of Morphology 266 (2): 125–66. doi:10.1002/jmor.10382. PMID 16217748. This theory is contested by most paleontologists.Prum, Richard O. (April 2003). "Are Current Critiques Of The Theropod Origin Of Birds Science? Rebuttal To Feduccia 2002". The Auk 120 (2): 550–61. The features cited as evidence of flightlessness are interpreted by mainstream paleontologists as exaptations, or "pre-adaptations", that maniraptorans inherited from their common ancestor with birds.
Protoavis texensis, was described in 1991 as a bird older than Archaeopteryx. Critics have indicated that the fossil is poorly preserved, extensively reconstructed, and may be a chimera (made up of fossilized bones from several different kinds of animals). The braincase is most likely that of a very early coelurosaurZhou, Zhonghe (October 2004). "The origin and early evolution of birds: discoveries, disputes, and perspectives from fossil evidence". Die Naturwissenschaften 91 (10): 455–71. doi:10.1007/s00114-004-0570-4.
Birds diversified into a wide variety of forms during the Cretaceous Period. Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors, the tails of more advanced birds were shortened with the advent of the pygostyle bone in the clade Pygostylia.
The first large, diverse lineage of short-tailed birds to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. More advanced lineages also specialised in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds"). Clarke, Julia A. (September 2004). "Morphology, Phylogenetic Taxonomy, and Systematics of Ichthyornis and Apatornis (Avialae: Ornithurae)". Bulletin of the American Museum of Natural History 286: 1–179. One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. Despite their extreme specialisations, the Hesperornithiformes represent some of the closest relatives of modern birds.
Containing all modern birds, the subclass Neornithes is, due to the discovery of Vegavis, now known to have evolved into some basic lineages by the end of the CretaceousClarke, Julia A.; Claudia P. Tambussi, Jorge I. Noriega, Gregory M. Erickson and Richard A. Ketcham (January 2005). "Definitive fossil evidence for the extant avian radiation in the Cretaceous". Nature 433: 305-308. doi:10.1038/nature03150. PMID 15662422. Supporting information and is split into two superorders, the Paleognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers) and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders and the guans and their allies). The dates for the splits are much debated by scientists. It is agreed that the Neornithes evolved in the Cretaceous, and that the split between the Galloanseri from other Neognathes occurred before the K–T extinction event, but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs.Ericson, Per G.P.; Cajsa L. Anderson, Tom Britton et al. (December 2006). "Diversification of Neoaves: Integration of molecular sequence data and fossils". Biology Letters 2 (4): 543–547. doi:10.1098/rsbl.2006.0523. PMID 17148284. This disagreement is in part caused by a divergence in the evidence; molecular dating suggests a Cretaceous radiation, while fossil evidence supports a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.Brown, Joseph W.; Robert B. Payne, David P. Mindell (June 2007). "Nuclear DNA does not reconcile \'rocks\' and \'clocks\' in Neoaves: a comment on Ericson et al.". Biology Letters 3 (3): 257-259. doi:10.1098/rsbl.2006.0611. PMID 17389215.
The classification of birds is a contentious issue. Sibley and Ahlquist\'s Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds,Sibley, Charles; Jon Edward Ahlquist (1990). Phylogeny and classification of birds. New Haven: Yale University Press. ISBN 0-300-04085-7. although it is frequently debated and constantly revised. Most evidence seems to suggest that the assignment of orders is accurate,Mayr, Ernst; Short, Lester L. (1970). Species Taxa of North American Birds/A Contribution to Comparative Systematics. Cambridge: Nuttal Orinthological Club. OCLC 517185. but scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.
based on Sibley-Ahlquist taxonomy |
This is a list of the taxonomic orders in the subclass Neornithes, or modern birds. This list uses the traditional classification (the so-called Clements order), revised by the Sibley-Monroe classification. The list of birds gives a more detailed summary of the orders, including families.
Subclass Neornithes
Paleognathae:
The radically different Sibley-Monroe classification (Sibley-Ahlquist taxonomy), based on molecular data, found widespread adoption in a few aspects, as recent molecular, fossil, and anatomical evidence supported the Galloanserae for example. By 2006, increasingly complete data had made it possible to verify the major proposals of the taxonomy, such as in Ciconiiformes, Gruiformes or Caprimulgiformes.
The range of the House Sparrow has expanded dramatically due to human activities.Newton, Ian (2003). The Speciation and Biogeography of Birds. Amsterdam: Academic Press, p. 463. ISBN 0-12-517375-X.
Birds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the Snow Petrel\'s breeding colonies up to 440 kilometres (270 mi) inland in Antarctica.Brooke, Michael (2004). Albatrosses And Petrels Across The World. Oxford: Oxford University Press. ISBN 0-19-850125-0. The highest bird diversity occurs in tropical regions. It was earlier thought that this high diversity was the result of higher speciation rates in the tropics, however recent studies found higher speciation rates in the high latitudes that were offset by greater extinction rates than in the tropics.Weir, Jason T.; Dolph Schluter (March 2007). "The Latitudinal Gradient in Recent Speciation and Extinction Rates of Birds and Mammals". Science 315 (5818): 1574–76. doi:10.1126/science.1135590. PMID 17363673. Several families of birds have adapted to life both on the world\'s oceans and in them, with some seabird species coming ashore only to breedSchreiber, Elizabeth Anne; Joanna Burger (2001). Biology of Marine Birds. Boca Raton: CRC Press. ISBN 0-8493-9882-7. and some penguins recorded diving up to 300 metres (980 ft).Sato, Katsufumi; Y. Naito, A. Kato et al. (May 2002). "Buoyancy and maximal diving depth in penguins: do they control inhaling air volume?". Journal of Experimental Biology 205 (9): 1189–1197. PMID 11948196.
Many bird species have established breeding populations in areas to which they have been introduced by humans. Some of these introductions have been deliberate; the Ring-necked Pheasant, for example, has been introduced around the world as a game bird.Hill, David; Peter Robertson (1988). The pheasant: Ecology, Management, and Conservation. Oxford: BSP Professional. ISBN 0-632-02011-3. Others have been accidental, such as the establishment of wild Monk Parakeets in several North American cities after their escape from captivity.Spreyer, Mark F.; Enrique H. Bucher (1998). Monk Parakeet (Myiopsitta monachus). The Birds of North America. Cornell Lab of Ornithology. DOI:10.2173/bna.322. Some species, including Cattle Egret,Arendt, Wayne J. (1988). "Range Expansion of the Cattle Egret, (Bubulcus ibis) in the Greater Caribbean Basin". Colonial Waterbirds 11 (2): 252–62. doi:10.2307/1521007. Yellow-headed CaracaraBierregaard, R.O. (1994). "Yellow-headed Caracara", in Josep del Hoyo, Andrew Elliott & Jordi Sargatal (eds.): Handbook of the Birds of the World. Volume 2; New World Vultures to Guineafowl. Barcelona: Lynx Edicions. ISBN 84-87334-15-6. and Galah,Juniper, Tony; Mike Parr (1998). Parrots: A Guide to the Parrots of the World. London: Christopher Helm. ISBN 0-7136-6933-0. have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.
External anatomy of a bird: 1 Beak, 2 Head, 3 Iris, 4 Pupil, 5 Mantle, 6 Lesser coverts, 7 Scapulars, 8 Median coverts, 9 Tertials, 10 Rump, 11 Primaries, 12 Vent, 13 Thigh, 14 Tibio-tarsal articulation, 15 Tarsus, 16 Feet, 17 Tibia, 18 Belly, 19 Flanks, 20 Breast, 21 Throat, 22 Wattle
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Compared with other vertebrates, birds have a body plan that shows many unusual adaptations, mostly to facilitate flight.
The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) which connect with the respiratory system.Ehrlich, Paul R.; David S. Dobkin, and Darryl Wheye (1988). Adaptations for Flight. Birds of Stanford. Standford University. Retrieved on 2007-12-13. Based on The Birder\'s Handbook (Paul Ehrlich, David Dobkin, and Darryl Wheye. 1988. Simon and Schuster, New York.) The skull bones are fused and do not show cranial sutures.Gill, Frank (1995). Ornithology. New York: WH Freeman and Co. ISBN 0-7167-2415-4. The orbits are large and separated by a bony septum. The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae."The Avian Skeleton", paulnoll.com. Retrieved on 2007-12-13. The last few are fused with the pelvis to form the synsacrum. The ribs are flattened and the sternum is keeled for the attachment of flight muscles except in the flightless bird orders. The forelimbs are modified into wings."Skeleton of a typical bird", Fernbank Science Center\'s Ornithology Web. Retrieved on 2007-12-13.
Like the reptiles, birds are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia. Uric acid is excreted along with feces as a semisolid waste since birds do not have a separate bladder or uretral opening.Ehrlich, Paul R.; David S. Dobkin, and Darryl Wheye (1988). Drinking. Birds of Stanford. Standford University. Retrieved on 2007-12-13.Tsahar, Ella; Carlos Martínez del Rio, Ido Izhaki and Zeev Arad (2005). "Can birds be ammonotelic? Nitrogen balance and excretion in two frugivores". Journal of Experimental Biology 208 (6): 1025–34. doi:10.1242/jeb.01495. PMID 15767304. However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.Preest, Marion R.; Carol A. Beuchat (April 1997). "Ammonia excretion by hummingbirds". Nature 386: 561–62. doi:10.1038/386561a0. They also excrete creatine, rather than creatinine like mammals. This material, as well as the output of the intestines, emerges from the bird\'s cloaca.Mora, J.; J. Martuscelli, Juana Ortiz-Pineda, and G. Soberón (1965). "The Regulation of Urea-Biosynthesis Enzymes in Vertebrates". Biochemical Journal 96: 28–35. PMID 14343146. Packard, Gary C. (1966). "The Influence of Ambient Temperature and Aridity on Modes of Reproduction and Excretion of Amniote Vertebrates". The American Naturalist 100 (916): 667–82. The cloaca is a multi-purpose opening: waste is expelled through it, birds mate by joining cloaca, and females lay eggs from it. In addition, many species of birds regurgitate pellets.Balgooyen, Thomas G. (1971). "Pellet Regurgitation by Captive Sparrow Hawks (Falco sparverius)". Condor 73 (3): 382–85. doi:10.2307/1365774. The digestive system of birds is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food to compensate for the lack of teeth.Gionfriddo, James P.; Louis B. Best (February 1995). "Grit Use by House Sparrows: Effects of Diet and Grit Size". Condor 97 (1): 57–67. doi:10.2307/1368983. Most birds are highly adapted for rapid digestion to aid with flight.Attenborough, David (1998). The Life of Birds. Princeton: Princeton University Press. ISBN 0-691-01633-X. Some migratory birds have the additional ability to reduce parts of the intestines prior to migration.Battley, Phil F.; Theunis Piersma, Maurine W. Dietz et als. (January 2000). "Empirical evidence for differential organ reductions during trans-oceanic bird flight". Proceedings of the Royal Society B 267 (1439): 191–5. doi:10.1098/rspb.2000.0986. PMID 10687826. (Erratum in Proceedings of the Royal Society B 267(1461):2567.)
Birds have one of the most complex respiratory systems of all animal groups. Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird\'s lungs receive a constant supply of fresh air during both inhalation and exhalation.Maina, John N. (November 2006). "Development, structure, and function of a novel respiratory organ, the lung-air sac system of birds: to go where no other vertebrate has gone". Biological Reviews 81 (4): 545–79. doi:10.1111/j.1469-185X.2006.tb00218.x. PMID 17038201. Sound production is achieved using the syrinx, a muscular chamber with several tympanic membranes which is situated at the lower end of the trachea, from where it separates.Suthers, Roderick A.; Sue Anne Zollinger (2004). "Producing song: the vocal apparatus", in H. Philip Zeigler & Peter Marler (eds.): Behavioral Neurobiology of Birdsong, Annals of the New York Academy of Sciences 1016. New York: New York Academy of Sciences, 109-129. DOI:10.1196/annals.1298.041. ISBN 1-57331-473-0. PMID 15313772 The bird\'s heart has four chambers and the right aortic arch gives rise to systemic circulation (unlike in the mammals where the left arch is involved). The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the red blood cells in birds have a nucleus.Scott, Robert B. (March 1966). "Comparative hematology: The phylogeny of the erythrocyte". Annals of Hematology 12 (6): 340–51. doi:10.1007/BF01632827. PMID 5325853.
The nervous system is large relative to the bird\'s size. The most developed part of the brain is the one that controls the flight-related functions, while the cerebellum coordinates movement and the cerebrum controls behaviour patterns, navigation, mating and nest building. Most birds have a poor sense of smell with notable exceptions including kiwis,Sales, James (2005). "The endangered kiwi: a review". Folia Zoologica 54 (1–2): 1–20. New World vulturesEhrlich, Paul R.; David S. Dobkin, and Darryl Wheye (1988). The Avian Sense of Smell. Birds of Stanford. Standford University. Retrieved on 2007-12-13. and tubenoses.Lequette, Benoit; Christophe Verheyden, Pierre Jouventin (August 1989). "Olfaction in Subantarctic seabirds: Its phylogenetic and ecological significance". The Condor 91 (3): 732–35. doi:10.2307/1368131. The avian visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water. Some species also have dual fovea. Birds are tetrachromatic, possessing ultraviolet (UV) sensitive cone cells in the eye as well as green, red and blue ones.Wilkie, Susan E.; Peter M. A. M. VISSERS, Debipriya DAS et als. (1998). "The molecular basis for UV vision in birds: spectral characteristics, cDNA sequence and retinal localization of the UV-sensitive visual pigment of the budgerigar (Melopsittacus undulatus)". Biochemical Journal 330: 541–47. PMID 9461554. This allows them to perceive ultraviolet light, which is involved in courtship. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet reflective patches on their feathers. Male Blue Tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.Andersson, S. (1998). "Ultraviolet sexual dimorphism and assortative mating in blue tits". Proceeding of the Royal Society B 265 (1395): 445–50. doi:10.1098/rspb.1998.0315. Ultraviolet light is also used in foraging—kestrels have been shown to search for prey by detecting the UV reflective urine trail marks left on the ground by rodents.Viitala, Jussi; Erkki Korplmäki, Pälvl Palokangas & Minna Koivula (1995). "Attraction of kestrels to vole scent marks visible in ultraviolet light". Nature 373 (6513): 425–27. doi:10.1038/373425a0. The eyelids of a bird are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally.Williams, David L.; Edmund Flach (March 2003). "Symblepharon with aberrant protrusion of the nictitating membrane in the snowy owl (Nyctea scandiaca)". Veterinary Ophthalmology 6 (1): 11–13. doi:10.1046/j.1463-5224.2003.00250.x. PMID 12641836. The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds. The bird retina has a fan shaped blood supply system called the pecten. Most birds cannot move their eyes, although there are exceptions, such as the Great Cormorant.White, Craig R.; Norman Day, Patrick J. Butler, Graham R. Martin (July 2007). "Vision and Foraging in Cormorants: More like Herons than Hawks?". PLoS ONE 2 (7): e639. doi:10.1371/journal.pone.0000639. PMID 17653266. Birds with eyes on the sides of their heads have a wide visual field, while birds with eyes on the front of their heads, such as owls, have binocular vision and can estimate the depth of field.Martin, Graham R.; Gadi Katzir (1999). "Visual fields in short-toed eagles, Circaetus gallicus (Accipitridae), and the function of binocularity in birds". Brain, Behaviour and Evolution 53 (2): 55–66. doi:10.1159/000006582. PMID 9933782. The avian ear lacks external pinnae but is covered by feathers, although in some birds, such as the Asio, Bubo and Otus owls, these feathers form tufts which resemble ears. The inner ear has a cochlea, but it is not spiral as in mammals.Saito, Nozomu (1978). "Physiology and anatomy of avian ear". The Journal of the Acoustical Society of America 64 (S1): S3. doi:10.1121/1.2004193.
A few species are able to use chemical defenses against predators; some Procellariiformes can eject an unpleasant oil against an aggressor,Warham, John (1977). "The Incidence, Function and ecological significance of petrel stomach oils". Proceedings of the New Zealand Ecological Society 24: 84–93. and some species of pitohuis from New Guinea secrete a powerful neurotoxin in their skin and feathers.Dumbacher,
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