Bird flight
Bird flight is the primary mode of locomotion used by most bird species in which birds take off and fly. Flight assists birds with feeding, breeding, avoiding predators, and migrating.
Bird flight includes multiple type of motion, including hovering, taking off, and landing, involves many complex movements. As different bird species adapted over millions of years through
Various theories exist about how bird flight evolved, including flight from falling or gliding (the trees down hypothesis), from running or leaping (the ground up hypothesis), from wing-assisted incline running or from proavis (pouncing) behavior.
Basic mechanics of bird flight
Lift, drag and thrust
The fundamentals of bird flight are similar to those of aircraft, in which the aerodynamic forces sustaining flight are lift, drag, and thrust. Lift force is produced by the action of air flow on the wing, which is an airfoil. The airfoil is shaped such that the air provides a net upward force on the wing, while the movement of air is directed downward. Additional net lift may come from airflow around the bird's body in some species, especially during intermittent flight while the wings are folded or semi-folded[1][2] (cf. lifting body).
Aerodynamic
Flight
Birds use mainly three types of flight, distinguished by wing motion.
Gliding flight
When in
Flapping flight
When a bird flaps, as opposed to gliding, its wings continue to develop lift as before, but the lift is rotated forward by the flight muscles to provide thrust, which counteracts drag and increases its speed, which has the effect of also increasing lift to counteract its weight, allowing it to maintain height or to climb. 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 thrust. At each up-stroke the wing is slightly folded inwards to reduce the energetic cost of flapping-wing flight.[4] Birds change the angle of attack continuously within a flap, as well as with speed.[5]
Bounding flight
Small birds often fly long distances using a technique in which short bursts of flapping are alternated with intervals in which the wings are folded against the body. This is a flight pattern known as "bounding" or "flap-bounding" flight.[6] When the bird's wings are folded, its trajectory is primarily ballistic, with a small amount of body lift.[2] The flight pattern is believed to decrease the energy required by reducing the aerodynamic drag during the ballistic part of the trajectory,[7] and to increase the efficiency of muscle use.[8][9]
Hovering
Several bird species use hovering, with one family specialized for hovering – the hummingbirds.[10][11] True hovering occurs by generating lift through flapping alone, rather than by passage through the air, requiring considerable energy expenditure.[10][12] This usually confines the ability to smaller birds, but some larger birds, such as a kite[13] or osprey[14][15] can hover for a short period of time. Although not a true hover, some birds remain in a fixed position relative to the ground or water by flying into a headwind. Hummingbirds,[11][12] kestrels, terns and hawks use this wind hovering.
Most birds that hover have high aspect ratio wings that are suited to low speed flying. Hummingbirds are a unique exception – the most accomplished hoverers of all birds.[10] Hummingbird flight is different from other bird flight in that the wing is extended throughout the whole stroke, which is a symmetrical figure of eight,[16] with the wing producing lift on both the up- and down-stroke.[11][12] Hummingbirds beat their wings at some 43 times per second,[17] while others may be as high as 80 times per second.[18]
Take-off and landing
Take-off is one of the most energetically demanding aspects of flight, as the bird must generate enough airflow across the wing to create lift. Small birds do this with a simple upward jump. However, this technique does not work for larger birds, such as albatrosses and swans, which instead must take a running start to generate sufficient airflow. Large birds take off by facing into the wind, or, if they can, by perching on a branch or cliff so they can just drop off into the air.
Landing is also a problem for large birds with high wing loads. 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, landing into wind and using their feet as skids. To lose height rapidly prior to landing, some large birds such as geese indulge in a rapid alternating series of
Wings
The bird's
Albatrosses have locking mechanisms in the wing joints that reduce the strain on the muscles during soaring flight.[20]
Even within a species wing morphology may differ. For example, adult
Female birds exposed to predators during ovulation produce chicks that grow their wings faster than chicks produced by predator-free females. Their wings are also longer. Both adaptations may make them better at avoiding avian predators.[22]
Wing shape
The shape of the wing is important in determining the flight capabilities of a bird. Different shapes correspond to different trade-offs between advantages such as speed, low energy use, and maneuverability. Two important parameters are the
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 slotted high-lift wings.[24]
Elliptical wings
Technically, elliptical wings are those having elliptical (that is quarter ellipses) meeting conformally at the tips. The early model Supermarine Spitfire is an example. Some birds have vaguely elliptical wings, including the albatross wing of high aspect ratio. Although the term is convenient, it might be more precise to refer to curving taper with fairly small radius at the tips. Many small birds have a low aspect ratio with elliptical character (when spread), allowing for tight maneuvering in confined spaces such as might be found in dense vegetation.[24] As such they are common in forest raptors (such as Accipiter hawks), and many passerines, 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 pheasants and partridges.
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 ducks. Birds that make long migrations typically have this type of wing.[24] The same wing shape is used by the auks for a different purpose; auks use their wings to "fly" underwater.
The peregrine falcon has the highest recorded dive speed of 242 miles per hour (389 km/h). The fastest straight, powered flight is the
High aspect ratio wings
High aspect ratio wings, which usually have low wing loading and are far longer than they are wide, are used for slower flight. This may take the form of almost hovering (as used by kestrels, terns and nightjars) or in soaring and gliding flight, particularly the dynamic soaring used by seabirds, which takes advantage of wind speed variation at different altitudes (wind shear) above ocean waves to provide lift. Low speed flight is also important for birds that plunge-dive for fish.
Soaring wings with deep slots
These wings are favored by larger species of inland birds, such as
Coordinated formation flight
A wide variety of birds fly together in a symmetric V-shaped or a J-shaped coordinated formation, also referred to as an "echelon", especially during long-distance flight or migration. It is often assumed that birds resort to this pattern of formation flying in order to save energy and improve the aerodynamic efficiency.
The wingtips of the leading bird in an echelon create a pair of opposite rotating line vortices. The vortices trailing a bird have an underwash part behind the bird, and at the same time they have an upwash on the outside, that hypothetically could aid the flight of a trailing bird. In a 1970 study the authors claimed that each bird in a V formation of 25 members can achieve a reduction of induced drag and as a result increase their range by 71%.[28] It has also been suggested that birds' wings produce induced thrust at their tips, allowing for proverse yaw and net upwash at the last quarter of the wing. This would allow birds to overlap their wings and gain Newtonian lift from the bird in front.[29]
Studies of waldrapp ibis show that birds spatially coordinate the phase of wing flapping and show wingtip path coherence when flying in V positions, thus enabling them to maximally utilise the available energy of upwash over the entire flap cycle. In contrast, birds flying in a stream immediately behind another do not have wingtip coherence in their flight pattern and their flapping is out of phase, as compared to birds flying in V patterns, so as to avoid the detrimental effects of the downwash due to the leading bird's flight.[30]
Adaptations for flight
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
The large amounts of energy required for flight have led to the evolution of a unidirectional pulmonary system to provide the large quantities of oxygen required for their high respiratory rates. This high metabolic rate produces large quantities of 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. [citation needed]
In addition to anatomical and metabolic modifications, birds have also adapted their behavior to a life in air. To avoid flying into each other, birds take to the right when they are on a collision course with other birds.[31]
Evolution of bird flight
Most paleontologists agree that birds evolved from small theropod dinosaurs, but the origin of bird flight is one of the oldest and most hotly contested debates in paleontology.[32] The four 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 feathersdeveloped for other reasons and then evolved further to provide first lift and then true powered flight.
- 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.
- Pouncing proavis, which posits that flight evolved by modification from arboreal ambush tactics.
There has also been debate about whether the earliest known bird, Archaeopteryx, could fly. It appears that Archaeopteryx had the
In March 2018, scientists reported that
From the trees down
This was the earliest hypothesis, encouraged by the examples of
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 that 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.[41]
From the ground up
Most recent attacks on the "from the ground up" hypothesis attempt to refute its assumption that birds are modified coelurosaurian dinosaurs. The strongest attacks are based on
Wing-assisted incline running
The
Pouncing proavis model
The proavis theory was first proposed by Garner, Taylor, and Thomas in 1999:
We propose that birds evolved from predators that specialized in ambush from elevated sites, using their raptorial hindlimbs in a leaping attack. Drag–based, and later lift-based, mechanisms evolved under selection for improved control of body position and locomotion during the aerial part of the attack. Selection for enhanced lift-based control led to improved lift coefficients, incidentally turning a pounce into a swoop as lift production increased. Selection for greater swooping range would finally lead to the origin of true flight.
The authors believed that this theory had four main virtues:
- It predicts the observed sequence of character acquisition in avian evolution.
- It predicts an Archaeopteryx-like animal, with a skeleton more or less identical to terrestrial theropods, with few adaptations to flapping, but very advanced aerodynamic asymmetrical feathers.
- It explains that primitive pouncers (perhaps like Microraptor) could coexist with more advanced fliers (like Confuciusornis or Sapeornis) since they did not compete for flying niches.
- It explains that the evolution of elongated rachis-bearing feathers began with simple forms that produced a benefit by increasing drag. Later, more refined feather shapes could begin to also provide lift.
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 migrate between the seasons. It is also used by some species to display during the breeding season[53] and to reach safe isolated places for nesting.
Flight is more energetically expensive in larger birds, and many of the largest species fly by soaring and gliding (without flapping their wings) as much as possible. Many physiological adaptations have evolved that make flight more efficient.
Birds that settle on isolated oceanic islands that lack ground-based predators may over the course of evolution lose the ability to fly. One such example is the flightless cormorant, native to the Galápagos Islands. This illustrates both flight's importance in avoiding predators and its extreme demand for energy.
See also
- Flight call
- Flying and gliding animals
- Insect flight
- List of soaring birds
- Ratites
- Tradeoffs for locomotion in air and water
- Patagium
Notes
- ^ "Intermittent Flight Studies". Flight Laboratory. The University of Montana-Missoula. Archived from the original on 10 March 2014.
- ^ a b Tobalske, B; et al. "The intermittent flight of Zebra Finches: Unfixed gears and body lift". Retrieved 6 March 2014.
- .
- ^ Parslew, B. (2012). Simulating Avian Wingbeats and Wakes, PhD Thesis
- PMID 21562173.
- ^ Bret W. Tobalske, Jason W. D. Hearn and Douglas R. Warrick, "Aerodynamics of intermittent bounds in flying birds", Exp. Fluids, 46, pp. 963–973 (2009), DOI 10.1007/s00348-009-0614-9 (accessed 2 August 2016)
- ^ Brendan Body, Tips and observations of bird flight: "Further affects of air resistance on small birds", 2009 (accessed 2 August 2016)
- PMID 10359676.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link - .
- ^ PMID 30263957.
- ^ PMID 29051535.
- ^ PMID 25767146.
- ^ Cascades Raptor Center (28 February 2012). "Cascades Raptor Center Show Behavior of the Year 2012". Archived from the original on 31 October 2021. Retrieved 31 March 2018 – via YouTube.
- ^ "Osprey General Information". www.newyorkwild.org. Retrieved 31 March 2018.
- ^ Wild West Nature (4 April 2013). "Osprey hovers like a hummingbird hunting in Yellowstone National Park". Archived from the original on 31 October 2021. Retrieved 31 March 2018 – via YouTube.
- PMID 17575042.
- PMID 22171086.
- ^ Gill V (30 July 2014). "Hummingbirds edge out helicopters in hover contest". BBC News. Retrieved 26 February 2019.
- ^ Baumel JJ (1993) Handbook of Avian Anatomy: Nomina Anatomica Avium. 2nd Ed. Nuttall Ornithological Club. Cambridge, MA, USA
- ISBN 0-19-856603-4pages 33-34
- S2CID 90387655.
- . Retrieved 27 March 2011.
- ^ "Wing aspect ratio". Science Learning Hub. Retrieved 20 March 2021.
- ^ a b c Lewis, Joe. "The Science of Flight in Relationship to Birds and Gliders". What Makes Airplanes Fly? History, Science and Applications of Aerodynamics. Yale-New Haven Teachers Institute. Retrieved 20 March 2021.
- ^ .
- ^ Batt, Bruce (1 October 2007). "Why do migratory birds fly in a V-formation?". Scientific American. Retrieved 16 January 2014.
- S2CID 4471158.
- S2CID 21251564.
- ^ On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds NASA
- S2CID 205237135.
- ^ "We Figured Out Why Birds Don't Fly Into Each Other". Popular Mechanics. 4 October 2016. Retrieved 21 August 2023.
- ISBN 978-0-684-84965-2.
- S2CID 4391019.
- ^ a b Senter, P. (2006). "Scapular orientation in theropods and basal birds, and the origin of flapping flight" (Automatic PDF download). Acta Palaeontologica Polonica. 51 (2): 305–313.
- ISBN 0-19-856603-4pages 98-117
- ^ Videler, John (1 January 2005). "How Archaeopteryx could run over water". Archaeopteryx. 23. Retrieved 31 March 2018 – via ResearchGate.
- PMID 29535376.
- ^ Guarino, Ben (13 March 2018). "This feathery dinosaur probably flew, but not like any bird you know". The Washington Post. Retrieved 13 March 2018.
- ISBN 978-0-300-07861-9. Archived from the originalon 2 June 2020. Retrieved 10 May 2012.
- S2CID 42829066.[permanent dead link]
- S2CID 535424.
- S2CID 6344830. Archived from the original(PDF) on 15 October 2003. Retrieved 11 April 2019.
- S2CID 28611454.
- S2CID 4430686.
- ISBN 978-0-7266-0287-0.
- ISBN 978-0-19-856603-8.
- . ScienceDaily. October 1997.
- .
- ]
- S2CID 82490156. Archived from the original(PDF) on 27 July 2011.
- . Scientists believe they could be a step closer to solving the mystery of how the first birds took to the air. BBC News. Retrieved 25 January 2008.
- S2CID 6323207.
- PMID 35440206.
References
- Alexander, David E. Nature's Flyers: Birds, Insects, and the Biomechanics of Flight. Baltimore: The Johns Hopkins University Press. ISBN 0801880599(paperback).
- Brooke, Michael and Tim Birkhead (editors). The Cambridge Encyclopedia of Ornithology. 1991. Cambridge: Cambridge University Press. ISBN 0521362059.
- Burton, Robert. Bird Flight. Facts on File, 1990
- Campbell, Bruce, and Elizabeth Lack (editors). A Dictionary of Birds. 1985. Calton: T&A D Poyse. ISBN 0856610399.
- Cornell Laboratory of Ornithology handbook of bird biology. 2004. Princeton University Press. ISBN 093802762X. (hardcover)
- Del Hoyo, Josep, et al. Handbook of Birds of the World Vol 1. 1992. Barcelona: Lynx Edicions, ISBN 8487334105.
- Wilson, Barry (editor). Readings from Scientific American, Birds. 1980. San Francisco: WH Freeman. ISBN 0716712067.
- Attenborough, D. 1998. The Life of Birds. Chapter 2. BBC Books. ISBN 0563387920.
External links
- 'Flight in Birds and Aeroplanes' by Evolutionary Biologist John Maynard Smith Freeview video provided by the Vega Science Trust
- Beautiful Birds in Flight - slideshow by Life magazine
- 'Pigeon Take off in slow motion' YouTube video
- 'Bird Flight I' Eastern Kentucky University ornithology course site, with pictures, text and videos.
- How do hummingbirds hover? Discovery
- Birds can transition between stable and unstable states via wing morphing on Nature, released in CC-BY