Bird flight

Flight is the mode of locomotion used by most of the world’s bird species. It is important to birds for feeding, breeding and avoiding predators.

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.

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 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 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 ).

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 to evade predators, such as pheasants and partridges.

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.

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. Some hummingbirds can beat their wings 52 times a second, others do so less frequently.

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.
• 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 0801867568(hardcover) and 0801880599(paperback).