Bird flight
From Wikipedia, the free encyclopedia.
Flight is the mode of locomotion used by most of the world’s bird species. It is important to birds for feeding, breeding and predator avoidance. Flight is possible because of the action of the wing as an airfoil. The action of the airfoil creates differences in air pressure below and above the wing, creating lift. The bird generates thrust to overcome drag by flapping its wing. The shape of the wing is important in determining the type of flight the bird is capable of. Flight is more energetically expensive in larger birds, and many of the largest species fly by soaring (gliding through the air without flapping their wings) most of the time. Birds have evolved many physiological adaptations in order to make flight more efficient.
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Evolution and purpose of bird flight
The origin of bird flight is still somewhat unclear, even though most paleontologists agree that birds evolved from small theropod dinosaurs. It seems likely that they evolved from ground living species, with flight developing after the evolution of feathers. It seems likely in this case that flight evolved in order to continue the pursuit of small airborne prey items (such as insects), possibly subsequently becoming useful as a predator avoiding behavior.
Today birds use flight for most possible purposes. It is still used by some species to obtain prey on the wing, as well as foraging, to commute to feeding grounds, and migrate between the seasons. Its importance in avoiding predators can be shown in the frequency with which it is lost when birds reach isolated oceanic islands that lack land predators. It is also used by some species to display during the breeding seasons and to reach safe isolated spots for nesting sites.
Basic mechanics of bird flight
The fundamentals of bird flight are very similar to that used by aircraft. For a bird to be able to fly, it must balance this against its weight, the drag and thrust. There are three types of drag that act against the thrust: frictional drag (the drag caused by the friction of air against the wing) parasite drag (the friction of the body against the wind) and induced drag (the drag caused by the vortexes caused by the air pressure imbalances). Unlike aircraft, where thrust is provided by engines, in birds thrust is provided by the wing as well, in the form of flapping. Flapping involves two stages, the downstroke, which provides the majority of the thrust, and the upstroke, which can (depending on the bird’s wings) provide some lift. At each upstroke the wing is slightly folded to reduce upward resistance. The wing produces lift at 90 degrees to the incident air, by moving the wing vertically the angle of incidents is changed. During the down stroke the lift is rotated forward and backward during the up stroke. The angle of attack is increased during the down stroke, this results in a nett forward force.
The wing
The bird's forelimbs, the wings, 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 (or metacarpo-digitals), one of two groups of feathers responsible for the airfoil shape. The other set of flight feathers that are behind the carpal joint on the ulna, are called the secondaries or cubitals. The remaining feathers on the wing are known a 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 Hoatzin), but claws are retained into adulthood by the Secretary Bird, the screamers and finfoots.
Wing shape and flight
The style of flight of any particular bird is dependent on the shape of the wing. This restricts the bird in some ways and enhances the bird in others. Wing shape can be described in terms of two parameters, aspect ratio and wing loading. Aspect ratio is the ratio of wing breadth to the mean of its chord, or mean wingspan divided by wing area. Wing loading is the ratio of weight to wing area.
Amongst the birds there are four main kinds of wing that the majority of birds use, although in some cases wings may fall between two of the categories. These types of wings are elliptical wings, high speed wings, high aspect ratio wings and soaring wings with slots.
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 passerines, particularly non-migratory ones (migratory species have longer wings). They are also common in species that use a rapid take off for defense, such as pheasants and partridges.
High speed wings
High speed wings are short, pointed wings that 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. The same wing shape is used by the auks for a different purpose; auks use their wings to "fly" underwater.
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, almost hovering (as used by kestrels, terns and nightjars) or alternatively by birds that specialize in soaring and gliding flight, particularly that used by seabirds, static soaring, which use different wind speeds at different heights above the waves in the ocean to provide thrust.
Soaring wings with deep slots
These are the wings favored by the larger species of inland birds, such as eagles, vultures, pelicans and storks. 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 ration wings require a long taxi in order to get airborne).
Hovering
Hovering is a demanding but useful ability used by several species of birds (and specialized in by one family). Hovering, literally 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.
Most birds that hover have high aspect ratio wings that are suited to low speed flying. One major exception to this are the hummingbirds, which are among the most accomplished hoverers of all the birds. Hummingbird flight is different to 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, others do so less frequently.
Take-off and landing
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 simply 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 airspeeds. 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 then the airspeed once the target is reached is virtually nil. Landing on water is simpler, and some species, such as swans, are only able to land on water.
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. The skeleton of the bird 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 vanes of the feathers have hooklets called barbules that zip them together, giving the feathers the strength needed to hold the airfoil (these are often lost in flightless birds).
References
- Del Hoyo, Josep, et al. Handbook of Birds of the World Vol 1. 1992. Barcelona: Lynx Edicions, ISBN 8487334105.
- Brooke, Michael and Tim Birkhead (editors). The Cambridge Encyclopedia of Ornithology. 1991. Cambridge: Cambridge University Press. ISBN 0521362059.
- Campbell, Bruce, and Elizabeth Lack (editors). A Dictionary of Birds. 1985. Calton: T&A D Poyse. ISBN 0856610399.
- Wilson, Barry (editor). Readings from Scientific American, Birds. 1980. San Francisco: WH Freeman. ISBN 0716712067.
