Bird flight 1837609 225652733 2008-07-14T19:14:16Z CarsracBot 6929011 robot Modifying: [[simple:Bird flight]] [[Image:Magpie Goose taking off.jpg|thumb|A [[magpie-goose]] taking off]] '''Flight''' is the main mode of [[animal locomotion|locomotion]] used by most of the world's '''bird''' species. [[Flight]] assists [[bird]]s while feeding, breeding and avoiding [[predation|predators]]. ==Evolution of bird flight== [[Image:Birds at Cape Hay.JPG|left|thumb|Marine birds fly at Cape Hay in the High [[Arctic]]]] Most [[paleontology|paleontologists]] agree that birds [[evolution|evolved]] from small [[Theropoda|theropod]] [[dinosaur]]s, but the origin of bird flight is one of the oldest and most hotly contested debates in paleontology.<ref name="Brush1998Taking Wing">{{ cite journal | author=Brush, A.H. | title=Taking Wing: Archaeopteryx and the Evolution of Bird Flight | journal=The Auk | date=July 1998 | url=http://findarticles.com/p/articles/mi_qa3793/is_199807/ai_n8788532}} Book review that provides a good, non-technical summary of the issues. The book is {{ cite book | author=Shipman, P. | title=Taking Wing: Archaeopteryx and the Evolution of Bird Flight | publisher=Simon & Schuster | isbn=0684849658}}</ref> The 3 main hypotheses are: "from the trees down", that birds' ancestors first glided down from trees and then acquired other modifications that enabled true powered flight; "from the ground up", that birds' ancestors were small, fast predatory dinosaurs in which [[feathers]] developed for other reasons and then evolved further to provide first lift and then true powered flight; and "wing-assisted incline running" (WAIR), a version of "from the ground up" in which birds' wings originated from forelimb modifications that provided ''downforce'', enabling the proto-birds to run up extremely steep slopes such as the trunks of trees. There has also been debate about whether the earliest known bird, ''[[Archaeopteryx]]'', could fly. It appears that ''Archaeopteryx'' had the brain structures and inner-ear balance sensors that birds use to control their flight.<ref name="AlonsoMilnerEtAl2004AvianBrainInnerEarArchaeopteryx">{{ cite journal | journal=Nature | volume=430 | issue=7000 | pages=666–669| date=August 2004 | doi=10.1038/nature02706 | url=http://www.nature.com/nature/journal/v430/n7000/full/nature02706.html | author=Alonso, P.D., Milner, A.C., Ketcham, R.A., Cokson, M.J and Rowe, T.B. | title=The avian nature of the brain and inner ear of Archaeopteryx }}</ref> ''Archaeopteryx'' also had a wing feather arrangement like that of modern birds and similarly asymmetrical flight feathers on its wings and tail. But ''Archaeopteryx'' lacked the [[Bird anatomy| shoulder mechanism]] by which modern birds' wings produce swift, powerful upstrokes; this may mean that it and other early birds were incapable of flapping flight and could only glide. <ref name="Senter2006ScapularOrientation" /> ===From the trees down=== This was the earliest hypothesis, encouraged by the examples of gliding vertebrates such as [[flying squirrel]]s. It suggests that proto-birds like ''[[Archaeopteryx]]'' used their claws to clamber up trees and glided off from the tops.<ref name="Feduccia1999OriginEvolutionBirds">{{ cite book | author=Feduccia, A. | title=The Origin and Evolution of Birds | date=1999 | publisher=Yale University Press | isbn=9780300078619 | url=http://yalepress.yale.edu/book.asp?isbn=9780300078619 }} See also {{ cite journal | author=Feduccia, A. | title=Explosive Evolution in Tertiary Birds and Mammals | journal=Science | volume=267 | issue=5198 | date=February 1995 | pages=637–638 | doi=10.1126/science.267.5198.637 | url=http://taxonomy.zoology.gla.ac.uk/~rdmp1c/teaching/L3/tutorials/feduccia/feduccia.html | pmid=17745839 }}</ref> Some recent research undermines the "trees down" hypothesis by suggesting that the earliest birds and their immediate ancestors did not climb trees. Modern birds that forage in trees have much more curved toe-claws than those which forage on the ground; the toe-claws of Mesozoic birds and of closely-related non-avian theropod dinosaurs are like those of modern ground-foraging birds. <ref name="GlenBennett2007ForagingModes">{{ cite journal | title=Foraging modes of Mesozoic birds and non-avian theropods | volume=17 | date=November 2007 | url=http://www.current-biology.com/content/article/abstract?uid=PIIS0960982207019859 | author=Glen, C.L., and Bennett, M.B. | format=abstract | unused_data=|Current Biology }}</ref> ===From the ground up=== [[Feathers]] are very common in [[coelurosaur| coelurosaurid dinosaurs]] (including the early tyrannosauroid [[Dilong]]).<ref name="prumbrush2002">{{ cite journal | date=2002 | title=The evolutionary origin and diversification of feathers | journal=The Quarterly Review of Biology | volume=77 | pages=261–295 | url=http://www.mcorriss.com/Prum_&_Brush_2002.pdf | author=Prum, R., and Brush, A.H. | doi=10.1086/341993 }}</ref> Modern [[bird]]s are classified as coelurosaurs by nearly all palaeontologists,<ref name="mayeretal2005">Mayr, G., B. Pohl & D.S. Peters (2005). "A well-preserved ''Archaeopteryx'' specimen with theropod features". ''Science'', '''310'''(5753): 1483-1486.</ref>, though not by a few [[ornithologists]].<ref name="Feduccia1999OriginEvolutionBirds">{{ cite book | author=Feduccia, A. | title=The Origin and Evolution of Birds | date=1999 | publisher=Yale University Press | isbn=9780300078619 | url=http://yalepress.yale.edu/book.asp?isbn=9780300078619 }} See also {{ cite journal | author=Feduccia, A. | title=Explosive Evolution in Tertiary Birds and Mammals | journal=Science | volume=267 | issue=5198 | date=February 1995 | pages=637–638 | doi=10.1126/science.267.5198.637 | url=http://taxonomy.zoology.gla.ac.uk/~rdmp1c/teaching/L3/tutorials/feduccia/feduccia.html | pmid=17745839 }}</ref><ref name="feduccia1993">Feduccia, A. (1993).</ref> The original functions of feathers may have included thermal insulation and competitive displays. The most common version of the "from the ground up" hypothesis argues that bird's ancestors were small ground-running predators (rather like [[roadrunners]]) that used their forelimbs for balance while pursuing prey and that the forelimbs and feathers later evolved in ways that provided gliding and then powered flight.<ref>{{ cite journal | date=1999 | title=The wing of Archaeopteryx as a primary thrust generator | journal=Nature | issue=399 | pages=60–62 | url=http://www.stephenjaygould.org/ctrl/news/file013.html | author=Burgers, P. and L. M. Chiappe }}</ref> Another "ground upwards" theory argues the evolution of flight was initially driven by competitive displays and fighting: displays required longer feathers and longer, stronger forelimbs; many modern birds use their wings as weapons, and downward blows have a similar action to that of flapping flight.<ref name ="CowenHistLife">{{ cite book | author=Cowen, R. | title=History of Life | publisher=Blackwell Science }}</ref> Many of the ''Archaeopteryx'' fossils come from marine sediments and it has been suggested that wings may have helped the birds run over water in the manner of the ''Jesus Christ Lizard'' ([[Common basilisk]]).<ref>{{ cirte book | author=Videler, J.J. | date=2005 | title=Avian Flight | publisher=Oxford University Press | location=Oxford }}</ref> Most recent attacks on the "from the ground up" hypothesis attempt to refute its assumption that birds are modified coelurosaurid dinosaurs. The strongest attacks are based on [[Embryology| embryological analyses]] which conclude that birds' wings are formed from digits 2, 3 and 4 (corresponding to the index, middle and ring fingers in humans; the first of a bird's 3 digits forms the [[Flight_feather#Alula | alula]], which they use to avoid [[Stall (flight)| stalling]] on low-speed flight, for example when landing); but the hands of coelurosaurs are formed by digits 1, 2 and 3 (thumb and first 2 fingers in humans).<ref>{{ cite journal | journal=Science | volume=278 | issue=666 date=1997 | title=Developmental patterns and the identification of homologies in the avian hand | doi=10.1126/science.278.5338.666 | url=http://www.sciencemag.org/cgi/content/abstract/278/5338/666?ijkey=dczDGiBvoF7W6 | author=Burke, A.C., and Feduccia, A. | year=1997 | pages=666 | format=abstract}} Summarized at {{ cite web | title=Embryo Studies Show Dinosaurs Could Not Have Given Rise To Modern Birds | url=http://www.sciencedaily.com/releases/1997/10/971027064254.htm | date=October 1997 | publisher=ScienceDaily}}</ref> However these embryological analyses were immediately challenged on the embryological grounds that the "hand" often develops differently in [[clades]] that have lost some digits in the course of their evolution, and therefore bird's hands do develop from digits 1, 2 and 3.<ref name="Chatterjee1998TechCommnetBurkeFeduccia">{{ cite journal | author=Chatterjee, S. | title=Counting the Fingers of Birds and Dinosaurs | journal=Science | date=April 1998 | volume=280 | issue=5362 | pages=355 | doi=10.1126/science.280.5362.355a | url=http://www.sciencemag.org/cgi/content/full/280/5362/355a }}</ref><ref name="">{{ cite journal | journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution | volume=304B | issue=1 | pages=86–90 | date=October 2004 | doi=10.1002/jez.b.21023 | url=http://www3.interscience.wiley.com/cgi-bin/abstract/109741948/ABSTRACT?CRETRY=1&SRETRY=0 | author=Vargas, A.O., Fallon, J.F. | title=Birds have dinosaur wings: The molecular evidence | format=abstract }}</ref><ref name="">{{ cite journal | title=Bird Wings Really Are Like Dinosaurs' Hands | journal=Science | date=January 2005 | volume=307 | pages=194–195 | doi=10.1126/science.307.5707.194b | url=http://www.ncsce.org/PDF_files/shift/Pennisi.pdf | author=Pennisi, E. | pmid=15653478 }}</ref> ===Wing-assisted incline running=== The WAIR hypothesis was prompted by observation of young [[chukar]] chicks, and proposes that wings developed their [[aerodynamic]] functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators. Note that in this scenario birds need ''downforce'' to give their feet increased grip.<ref name="Dial2003WAIR">{{ cite journal | author=Dial, K.P. | date=2003 | title=Wing-Assisted Incline Running and the Evolution of Flight | journal=Science | volume=299 | issue=5605 | pages=402–404 | doi=10.1126/science.1078237 | url=http://www.sciencemag.org/cgi/content/abstract/299/5605/402 | format=abstract | pmid=12532020 }} Summarized in {{cite web | last =Morelle | first =Rebecca | title =Secrets of bird flight revealed | work =Scientists believe they could be a step closer to solving the mystery of how the first birds took to the air. | publisher = BBC News | date =Thursday, 24 January 2008 | url =http://news.bbc.co.uk/2/hi/science/nature/7205086.stm | format =Web | doi = | accessdate = 2008-01-25 }}</ref><ref name="BundleDial2003MechanicsOfWAIR">{{ cite journal | date=2003 | title=Mechanics of wing-assisted incline running (WAIR) | journal=The Journal of Experimental Biology | volume=206 | pages=4553–4564 |url=http://dbs.umt.edu/flightlab/pdf/bundle%20and%20dial%20JEB%202003.pdf | author=Bundle, M.W and Dial, K.P. | doi=10.1242/jeb.00673 | pmid=14610039 }}</ref> But early birds, including ''[[Archaeopteryx]]'', lacked the [[Bird anatomy| shoulder mechanism]] by which modern birds' wings produce swift, powerful upstrokes; since the downforce on which WAIR depends is generated by upstrokes, it seems that early birds were incapable of WAIR.<ref name="Senter2006ScapularOrientation">{{ cite journal | author=Senter, P. | date=2006 | title=Scapular orientation in theropods and basal birds, and the origin of flapping flight | journal=Acta Palaeontologica Polonica | volume=51 | issue=2 | pages=305–313 | url=http://www.app.pan.pl/acta51/app51-305.pdf | format={{dead link|date=July 2008}} &ndash; <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=intitle%3AScapular+orientation+in+theropods+and+basal+birds%2C+and+the+origin+of+flapping+flight&as_publication=Acta+Palaeontologica+Polonica&as_ylo=&as_yhi=&btnG=Search Scholar search]</sup> }}</ref> ===Uses and loss of flight in modern birds=== Birds use flight to obtain prey on the wing, for [[foraging]], to commute to feeding grounds, and to [[bird migration|migrate]] between the seasons. It is also used by some species to display during the breeding season and to reach safe isolated places for [[nest]]ing. Flight is more energetically expensive in larger birds, and many of the largest species fly by [[Flying and gliding animals|soaring and gliding]] (without flapping their wings) most of the time. Many physiological adaptations have evolved that make flight more efficient. Birds that settle on isolated [[List of islands|oceanic island]]s that lack ground-based predators often lose the ability to fly. This illustrates both flight's importance in avoiding predators and its extreme demand for energy. ==Basic mechanics of bird flight== [[Image:Lift and angle of attack.jpg|thumb|The lift force has both a forward and a vertical component.]] The fundamentals of bird flight are similar to those of [[aircraft]]. [[Lift (force)|Lift force]] is produced by the action of air flow on the [[wing]], which is an [[airfoil]]. The lift force occurs because the air has a lower [[air pressure|pressure]] just above the wing and higher pressure below. When [[gliding]], both birds and [[glider]]s obtain both a vertical and a forward force from their wings. This is possible because the lift force is generated at right angles to the air flow, which in level flight comes from slightly below the wing. The lift force therefore has a forward component. [[Image:Forces in flight.jpg|thumb|Forces acting on a wing]] When a bird flaps, as opposed to gliding, its wings continue to develop lift as before, but they also create an additional forward and upward force, [[thrust]], to counteract its [[weight]] and [[drag (physics)|drag]]. Flapping involves two stages: the down-stroke, which provides the majority of the thrust, and the up-stroke, which can also (depending on the bird's wings) provide some upward force. At each up-stroke the wing is slightly folded inwards to reduce upward resistance. Birds change the [[angle of attack]] between the up-stroke and the down-stroke of their wings. During the down-stroke the angle of attack is increased, and is decreased during the up-stroke. There are three major forces that impede a bird's aerial flight: [[skin friction|frictional drag]] (caused by the friction of air and body surfaces), [[form drag]] (due to frontal area of the bird, also known as pressure drag), and [[lift-induced drag]] (caused by the [[wingtip vortices]]). ==The wing== [[Image:Kea in Flight MC.jpg|thumb|left|A [[kea]] in flight]] The bird's [[forelimb]]s, the [[wing]]s, are the key to bird flight. Each wing has a central vane to hit the wind, composed of three limb bones, the [[humerus]], [[ulna]] and [[radius]]. The hand, or manus, which ancestrally was composed of five digits, is reduced to three digits (digit II, III and IV), the purpose of which is to serve as an anchor for the primaries, one of two groups of [[flight feather]]s responsible for the wing's airfoil shape. The other set of flight feathers, which are behind the carpal joint on the ulna, are called the secondaries. The remaining feathers on the wing are known as [[covert (feather)|coverts]], of which there are three sets. The wing sometimes has vestigial claws. In most species these are lost by the time the bird is adult (such as the highly visible ones used for active climbing by [[Hoatzin]] chicks), but claws are retained into adulthood by the [[Secretary Bird]], [[screamer]]s, [[Heliornithidae|finfoot]]s, ostriches, several swifts and numerous others, as a local trait, in a few specimens. The claws of the Jurassic theropod-like [[Archaeopteryx]] are quite similar to those of the Hoatzin nestlings. ==Wing shape and flight== [[Image:Male mallard flight - natures pics.jpg|thumb|A male [[Mallard]] in flight.]] The shape of the wing is an important factor in determining the types of flight of which the bird is capable. Different shapes correspond to different trade-offs between beneficial characteristics, such as speed, low energy use, and maneuverability. The [[planform]] of the wing (the shape of the wing as seen from below) can be described in terms of two parameters, [[aspect ratio (wing)|aspect ratio]] and [[wing loading]]. Aspect ratio is the ratio of wing breadth to the mean of its [[chord (aircraft)|chord]], or mean [[wingspan]] divided by wing area. Wing loading is the ratio of weight to wing area. Most kinds of bird wing can be grouped into four types, with some falling between two of these types. These types of wings are elliptical wings, high speed wings, high aspect ratio wings and soaring wings with slots. [[Image:FlightSilhouettes.svg|thumb|left|Wing shapes]] ===Elliptical wings=== Elliptical wings are short and rounded, having a low aspect ratio, allowing for tight maneuvering in confined spaces such as might be found in dense vegetation. As such they are common in forest raptors (such as ''[[Accipiter]]'' hawks), and many [[passerine]]s, particularly non-migratory ones (migratory species have longer wings). They are also common in species that use a rapid take off to evade predators, such as [[pheasant]]s and [[partridge]]s. ===High speed wings=== High speed wings are short, pointed wings that when combined with a heavy wing loading and rapid wingbeats provide an energetically expensive high speed. This type of flight is used by the bird with the fastest wing speed, the [[peregrine falcon]], as well as by most of the [[duck]]s. The same wing shape is used by the [[auk]]s for a different purpose; auks use their wings to "fly" underwater. The Peregrine Falcon has the highest recorded dive speed of 175 mph (282 km/h). The fastest straight, powered flight is the [[Spine-tailed Swift]] at 105 mph (170 km/h). ===High aspect ratio wings=== [[Image:RoseateTern23.jpg|thumb|right|This [[Roseate Tern]] uses its low wing loading and high aspect ratio to achieve low speed flight, important for a species that plunge dives for fish]] High aspect ratio wings, which usually have low wing loading and are far longer than they are wide, are used for slower flight, almost hovering (as used by [[kestrel]]s, [[tern]]s and [[nightjar]]s) or alternatively by birds that specialize in soaring and [[gliding]] flight, particularly that used by [[seabird]]s, <!--static?-->[[dynamic soaring]], which use different wind speeds at different heights ([[wind shear]]) above the waves in the ocean to provide lift. ===Soaring wings with deep slots=== These are the wings favored by the larger species of inland birds, such as [[eagle]]s, [[vulture]]s, [[pelican]]s, and [[stork]]s. The slots at the end of the wings, between the primaries, reduce the [[turbulence]] at the tips, whilst the shorter size of the wings aids in takeoff (high aspect ratio wings require a long [[taxiing|taxi]] in order to get airborne). ==Hovering== [[Levitation|Hovering]] is used by several species of birds (and specialized in by one family). Hovering, which is generating lift through flapping alone rather than as a product of thrust, demands a lot of energy. This means that it is confined to smaller birds; the largest bird able to truly hover is the [[pied kingfisher]], although larger birds can hover for small periods of time. Larger birds that hover do so by flying into a headwind, allowing them to utilize thrust to fly slowly but remain stationary to the ground (or water). Kestrels, terns and even hawks use this windhovering. [[Image:IMG 42371h.jpg|thumb|left|The [[ruby-throated Hummingbird]] can beat its wings 52 times a second]] Most birds that hover have high aspect ratio wings that are suited to low speed flying. One major exception to this are the [[hummingbird]]s, which are among the most accomplished hoverers of all the birds. Hummingbird flight is different from other bird flight in that the wing is extended throughout the whole stroke, the stroke being a symmetrical figure of eight, with the wing being an airfoil in both the up- and down-stroke. Some hummingbirds can beat their wings 52 times a second, though others do so less frequently. ==Take-off and landing== [[Image:Male Bufflehead taking off.jpg|thumb|A male [[bufflehead]] runs atop the water while taking off]] Take-off can be one of the most energetically demanding aspects of flight, as the bird needs to generate enough airflow under the wing to create lift. In small birds a jump up will suffice, while for larger birds this is not possible. In this situation, birds need to take a run up in order to generate the airflow to take off. Large birds often simplify take off by facing into the wind, and, if they can, perching on a branch or cliff so that all they need to do is drop off into the air. Landing is also a problem for many large birds with high [[airspeed]]s. This problem is dealt with in some species by aiming for a point below the intended landing area (such as a [[nest]] on a cliff) then pulling up beforehand. If timed correctly, the airspeed once the target is reached is virtually nil. Landing on water is simpler, and the larger waterfowl species prefer to do so whenever possible. In order to lose height rapidly prior to landing, some large birds such as geese indulge in a rapid alternating series of [[Slip (aerodynamic)|sideslips]] in a maneuver termed as ''whiffling''. [[Image:mute swan flies arp.jpg|thumb|left|Mute Swan ''Cygnus olor'']] ==Adaptations for flight== [[Image:Underwing.svg|thumb|1 Axillaries; 2 Margin (Marginal underwing coverts); 3 Lesser underwing coverts; 4 Median underwing coverts (Secondary coverts); 5 Greater underwing coverts (Secondary coverts); 6 Carpal joint; 7 Lesser underwing primary coverts; 8 Greater undering primary coverts; 9 Secondaries; 10 Primaries]] The most obvious adaptation to flight is the wing, but because flight is so energetically demanding birds have evolved several other adaptations to improve efficiency when flying. Birds' bodies are streamlined to help overcome air-resistance. Also, the [[bird skeleton]] is hollow to reduce weight, and many unnecessary bones have been lost (such as the bony tail of the early bird ''[[Archaeopteryx]]''), along with the toothed jaw of early birds, which has been replaced with a lightweight [[beak]]. The skeleton's breastbone has also adapted into a large keel, suitable for the attachment of large, powerful flight muscles. The vanes of the [[feather]]s have hooklets called [[barbule]]s that zip them together, giving the feathers the strength needed to hold the airfoil (these are often lost in [[flightless bird]]s). The large amounts of energy required for flight have led to the evolution of a [[Bird_anatomy#Respiratory_system|unidirectional pulmonary system]] to provide the large quantities of oxygen required for their high [[respiration rate]]s. This high [[metabolism|metabolic rate]] produces large quantities of [[radical (chemistry)|radicals]] in the cells that can damage DNA and lead to tumours. Birds, however, do not suffer from an otherwise expected shortened lifespan as their cells have evolved a more efficient antioxidant system than those found in other animals. ==See also== *[[Insect flight]] *[[List of soaring birds]] *[[Flying and gliding animals]] ==Notes== {{reflist}} ==References== *Del Hoyo, Josep, et al. ''Handbook of Birds of the World Vol 1''. 1992. Barcelona: Lynx Edicions, ISBN 84-87334-10-5. *Brooke, Michael and Tim Birkhead (editors). ''The Cambridge Encyclopedia of Ornithology''. 1991. Cambridge: Cambridge University Press. ISBN 0-521-36205-9. *Campbell, Bruce, and Elizabeth Lack (editors). ''A Dictionary of Birds''. 1985. Calton: T&A D Poyse. ISBN 0-85661-039-9. *Wilson, Barry (editor). ''Readings from Scientific American, Birds''. 1980. San Francisco: WH Freeman. ISBN 0-7167-1206-7. *Alexander, David E. ''Nature's Flyers: Birds, Insects, and the Biomechanics of Flight''. 2002(hardcover) and 2004(paperback). Baltimore: The Johns Hopkins University Press. ISBN 0-8018-6756-8(hardcover) and 0801880599(paperback). *Cornell Laboratory of Ornithology ''handbook of bird biology''. 2004(hardcover). Princeton University Press. ISBN 0-938-02762-x(hardcover). ==External links== *[http://www.vega.org.uk/video/programme/84 'Flight in Birds and Aeroplanes' by Evolutionary Biologist John Maynard Smith] Freeview video provided by the Vega Science Trust *[http://www.youtube.com/watch?v=nzp5L-U00p8 'Pigeon Take off in slow motion'] You Tube video *[http://people.eku.edu/ritchisong/554notes2.html 'Bird Flight I'] Eastern Kentucky University ornithology course site, with pictures, text and videos. {{Footer Birds}} [[Category:Ornithology|Flight]] [[Category:Birds|Flight]] [[Category:Aerodynamics]] [[Category:Locomotion]] [[ca:Vol dels ocells]] [[de:Fliegen (Fortbewegung)#Vogelflug]] [[es:Vuelo de las aves]] [[nl:Vogelvlucht]] [[simple:Bird flight]]