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 as a result of benefits in the pursuit of small airborne prey items (such as insects), possibly subsequently becoming useful as a predator avoiding behavior.
Flight is more energetically expensive in larger birds, and many of the largest species fly by soaring (gliding without flapping their wings) most of the time. Many physiological adaptations have evolved that make flight more efficient.
Today birds use flight for many 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. Flight's importance in avoiding predators can be shown in the frequency with which it is lost when birds reach isolated oceanic islands that lack ground-based predators. It is also used by some species to display during the breeding season and to reach safe isolated places for nesting.
Basic mechanics of bird flight
The fundamentals of bird flight are similar to those of aircraft. Lift force is produced by the action of air-flow on the wing, which is an airfoil/aerofoil. The lift-force is because the air has a lower air pressure just above the wing and higher pressure below.
When gliding, both birds and gliders 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. (Weight always acts vertically downwards and so cannot provide a forward force. Without a forward component a gliding bird would merely descend vertically.)
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. 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-strokes and the down-strokes 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: 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 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 finfoot.
Wing shape and flight
The shape of the wing is important in determining the type of flight of which the bird is capable, planform. 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 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 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. The same wing shape is used by the auks for a different purpose; auks use their wings to "fly" underwater.
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 ratio wings require a long taxi in order to get airborne).
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 the larger waterfowl species prefer to do so whenever possible.
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 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 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).
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 respiration 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.
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- 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).
- 'Flight in Birds and Aeroplanes' by Evoluntionary Biologist John Maynard Smith Freeview video provided by the Vega Science Trust.