TEXTΦC Principles of FlightThe flight of birds, bats, airplanes, helicopters--even the surface-borne ╥flight╙ of boats sailing to weather--rely on the same phenomenon of ╥lift.╙ Aerodynamic lift results from differentials in air pressure on two sides of a streamlined structure with its leading edge facing into an airstream. Consider a cross section of a wing, or a tightly trimmed sail. Air flows farther--and hence faster--over the curved upper surface of the wing than does the air traveling over the flatter under surface. With higher velocity, the air over the wing exerts less pressure than the air beneath the wing, resulting in lift. The pressure differential can be changed by tilting the leading edge of the wing upward or downward. This angle is called the ╥angle of attack╙ and is usually about 3-5 degrees from horizontal in the normal flight of most birds. If the angle of attack is tilted too strongly upward, lift is reduced, then lost. Such loss of lift is called ╥stall.╙When a bird is planning to alight, it raises the angle of attack higher and higher as it descends. The resulting loss of lift reduces airspeed and increases drag. Within a few inches of the ground, the angle of attack has reached about 25-30 degrees, the wings stall, and the bird touches down. Drag and slowing can be increased by dropping the tail as well.The principle of lift, resulting from differential air pressures on an airfoil, is basic to the function of a wing. But the full dynamics of bird flight are far more complicated. A bird╒s wing is not a fixed airfoil. Rather, it is a dynamic complex of many airfoils under constant adjustment.Turbulence is created where the faster moving air passing over the wing meets the slower air coming under the wing. The up-flowing air from beneath the wing is especially strong at the tip of the wing. This so-called ╥wing-tip vortex╙ affects the efficient flow of air and causes a loss of flying efficiency. Birds with longer wingspans separate the vortex from the lifting area of the inner wing and hence increase their efficiency. This is seen clearly in birds with high-aspect-ratio wings (greater length compared to width) like albatrosses and frigatebirds.Lifting power may also be increased by slots in the wing. The rate of air flow is increased because air must travel through a restricted space, the slot. Slots are well-developed in birds with stouter wings, such as crows, jays, wood warblers, catbirds, and fowl-like birds.FeathersFeathers serve a number of functions for birds, but flight support is chief among them. It is possible that the original primary function of feathers, in the early evolutionary history of warm-blooded birds, was insulation--a function, of course, they still serve. The arrangement and coloration of feathers are also functional in communication (e.g., the red shoulder patches, or epaulettes, of Red-winged Blackbirds in defense of territories) and courtship (e.g., cranes, albatrosses).Individual birds have a variety of feather types, from fluffy down to stiff bristles. For flight, the key feather types are primary, secondary, alular (wing-tip), retricial (tail feathers), and contour feathers. Contour feathers form the outline of the body and are the principal body covering. They help reduce drag in flight.Primary flight feathers are attached to the hand, or ╥manus.╙ Their number is constant within bird groups and varies only slightly among major groups. Flying birds have between nine and twelve primary feathers on each wing, although the outermost may be reduced in size. Most bird species have ten primary feathers per wing. Some North American passerine groups have nine primaries (e.g., swallows, wood warblers, tanagers, and finches).Secondary flight feathers are attached to the bird╒s forearm, or ╥ulna.╙ Their numbers range more widely among bird groups than do the numbers of primaries. The smallest numbers are found in hummingbirds, whose very greatly reduced ulnas bear six to seven secondaries. Most passerines have nine. A sample of some North American birds gives an idea of the range of numbers in secondary feathers on each wing:bobwhite 10woodpecker 11 kingfisher 12Great Horned Owl 14Cooper╒s Hawk 15Wild Turkey 16Red-tailed Hawk 17Turkey Vulture 18Mallard 19Osprey 20California Condor 22albatross 32Primary and secondary feathers are sometimes referred to collectively as ╥remiges.╙ Tail feathers are called ╥rectrices.╙ Most birds have 6 pairs; some have five (e.g., hummingbirds, swifts). Other groups have more: twelve pairs in white pelicans; nine pairs in Prairie-Chickens.Flight MusclesThe driving power for wing movement--the ╥motor╙ of all flapping birds--stems from the powerful breast or pectoral muscles. The flight muscles of the strongest fliers comprise about 15-25 percent of the bird╒s total weight. The relatively enormous breast muscles of the Ruby-throated Hummingbird account for about 30 percent of its weight and permit wingbeat rates of up to 70 beats a second in males and 50 beats per second in females.The WingA bird╒s wing is homologous to our human arm, wrist, and hand. It consists of two functional parts:1) An inner part nearest the body is moved by the bird╒s shoulder joint and bears the secondary flight feathers. The inner part of the wing almost exclusively provides the bird with lift. In flight, whether soaring, gliding, or flapping, the bird holds this part of the wing out rather rigidly from its sides, with a slight tilt upward and forward (the angle of attack) like the rigid wing of an airplane.2) An outer part--the ╥hand╙ section--is moved separately by the bird╒s wrist, the joint at the outer bend of its wing. This outer section, with strong, pliable primary flight feathers, functions as a propeller when the bird beats the air in flapping flight.Reduced WeightBirds have evolved a number of adaptations or strategies for reducing weight, resulting in more efficient flight. These include the loss of the right ovary in most birds, the resorption of marrow from long bones and their replacement with air sacs, and the fusion of many bones into lighter and stronger support for muscles.Flapping Flight--Using the ╥Propellers╙In free flight, the bird╒s powerful breast muscles (the pectorals) sweep the whole wing up and down from the shoulder. The inner part of the wing, although it does not actually need to move for flapping flight, acts as a handle for the propeller (outer part of wing) and gives it greater speed and power.A bird has almost complete control over the hand section of its wings, just as a human has over his or her hand and fingers. The bird can twist its hand (the outer wing primaries) to any position, waggle them about, and can even clap the outer wings together behind its head and in front of its breast.In normal flapping flight, the wings of a bird sweep through the downbeat fully extended. At the end of the downstroke, the bird flexes its ╥wrist╙ to begin the upsweep of the wings. The whole wing then starts upward, the hand section first. The primary or flight feathers of the hand section partially separate like the fingers of a hand. The inner half of the wing pulls along after the outer hand section. The outer part is held vertically rather than horizontally as on the down-stroke. At the end of the upward sweep of the wings, the hand section, in a sudden powerful burst, flaps up and out and resumes its position for the next downward sweep of the wings.This series of changes in the angles of attack of primary feathers, and the sections of the wing while the wing is in motion, creates propulsive lift in a way similar to the propulsive force of an airplane╒s propeller.Gliding FlightGliding, the most elementary form of flight, is used by almost all birds, except hummingbirds. A bird╒s wings make no propulsive movements in gliding. Although it may adjust the angle of the wing, a gliding bird is actually coasting ╥downhill╙ in relation to the flow of air. Gliding in still air means a loss of altitude for a bird. To regain it, the bird must either resume flapping flight or find rising currents of air on which to soar aloft again.Soaring FlightA soaring bird maintains or even increases its altitude without flapping its wings. Vultures that soar over land, and albatrosses over oceans, have become so highly adapted to soaring that they use this method of flight almost exclusively.The ╥static soarers╙ include buteonine hawks, vultures, eagles, and condors. They keep aloft mainly by riding rising currents of warm air called thermals. Soaring birds also use obstruction currents. These are updrafts of air caused when steady winds are deflected by objects such as mountains, hills, buildings, sand dunes, and ocean waves.Hovering FlightMany kinds of birds hover; that is, they maintain their position in midair by flapping and rotating their wings sufficiently to maintain altitude without moving forward. Certain hawks and kingfishers are notably expert at it as they hover in the air to watch for their prey directly below them. Kingfishers and kestrels hold position in the air by beating their wings up and down and depressing and spreading their tail feathers, with their body held almost vertically.Hummingbirds are masters of hovering flight. Their flight method has been likened to the circular whirl of a helicopter╒s rotor, allowing it to hover, to move ahead, backward, or sideways. This unique flying pattern depends on the distinctive design of the hummingbird╒s wing.All birds except hummingbirds move their wings at the shoulder, elbow, and wrist. They really fly from the wrist out, on the outer wing with their long flight feathers. In contrast, a hummingbird moves its wings completely from its shoulders. This gives the wings their astonishingly free, propeller-like movements, and the hummingbird its maneuverability.Formation FlyingThe wing of a flying bird creates behind it a small area of disturbed air, the ╥slip-stream.╙ As each bird flies, some air is lost over the wing tips, causing a loss of lift to the bird. This circulation of air creates an enlarging spiral wing-tip vortex behind each wing tip, with upswelling air on the outer side of each wing. Wild geese and swans, by flying in formation, use the upswelling air created by their flock mates to save their energy when flying.In the V formation of Canada Geese, for example, each bird flies not directly behind another, but beside or above the bird in front. By so doing each bird rests its inner wing-tip in flight on the rising vortex of air from the bird╒s wing in front of it. This energy-saving V flight may give birds that use it 70 percent more flight distance than the range attained by a lone goose.Courtship and Territorial FlightSome birds have spectacular courtship and territorial flights, usually practiced by the male alone. Sometimes, however, he is joined by the female--for example, in both the Red-tailed Hawk and the Golden Eagle. The male Peregrine Falcon has one of the most dashing of all courtship flights, in which he may perform a series of mile-long dives, aerial somersaults, barrel rolls, and ╥figure eights╙ at breathtaking speed. These are usually executed by the male over the nesting cliff of the mated pair.The leading edge of each wing (the outer primary feather) of owls has fine serrations like the cutting edge of a saw. This ragged edge silences the noise of the vortex of air rushing over the flying owl╒s wings; but during courtship at least one of the larger owls deliberately makes loud noises with its wings. While soaring about at dusk in its courtship flights, the Short-eared Owl, after uttering a series of low tooting sounds, suddenly plunges earthward in a shallow dive. This is accompanied by a clapping sound of its wings as it stretches them in back of its body and slaps them together in short strokes.Non-Aerial ╥Flight╙True flight, of course, happens in the air. However, the same adaptations and muscle groups and skeletal frameworks support the underwater ╥flight╙ of diving birds like penguins and alcids (e.g., puffins, murres). Wings serve important functions for flightless birds as well. Running bi-pedal birds such as ostriches and emus use their wings for balancing. Flightless cormorants in the Galapagos Islands use spread wings to control their body temperature.Time Spent in FlightSwifts and swallows apparently spend more time in flight than any other land birds. A banded Chimney Swift that had lived for nine years was estimated to have flown 1,350,000 miles during its lifetime. This included nine round trips from the U.S. to wintering grounds in South America. The Black-capped Chickadee flies about 62 miles a day while traveling back and forth to feed its nestlings.Albatrosses spend more time in the air than any other birds, soaring on their very specialized wing structure. Spanning 11 feet from tip to tip, but only about 9 inches from front to back, the narrow high-aspect-ratio wings of these birds are the superb ╥sails.╙ Albatrosses spend most of their lives in the air, and even sleep on the wing.How High Do Birds Fly?Except during migration, most birds ordinarily fly 50-100 feet above the ground. Of course, if birds are living at high altitudes, e.g., in mountains, their actual flying altitude can be high indeed.The highest altitude records have come from the high Alps and Himalayas, barrier ranges that must be crossed by migrating birds. Fast, long-winged shorebirds such as godwits and curlews have been seen flying past Mt. Everest at 21,000 feet. Migrating Bar-headed Geese (╥Anser indicas╙) have been reported flying over the summit of the Himalayan peaks, 27,824 feet above sea level. In 1973 an airplane collided with a Ruppell╒s Griffon (╥Gyps rueppelli╙) over West Africa at an altitude of 37,000 feet.Many high altitude records of North American birds have been observed from airplane sightings or collisions. The highest such record for a North American bird resulted from the mid-air collision of a commercial jet-liner with a Mallard at 21,000 feet over Elko, Nevada, in 1963. Other North American high fliers noted via aircraft include Chimney Swifts over Lufkin, Texas, at 7,000 feet; a Golden-crowned Sparrow over central California at 10,000 feet; and two Evening Grosbeaks near Boulder, Colorado, at 12,500 feet.Birds of prey, such as eagles, hawks, and falcons, often fly hundreds of feet over the ground to spot mammals or low-flying birds, and then drop down to capture these prey items. Vultures and condors soar to altitudes of thousands of feet to survey the ground for carrion, or to spot other vultures circling and descending to feed on dead animals.How Fast Do Birds Beat Their Wings?Wingbeat rates (in wingbeats per second) measured by observers in wind tunnels and using photography include:Black-capped Chickadee 27Northern Mockingbird 14Ruby-throated Hummingbird male 70 female 50Double-crested Cormorant 2.6Black Duck 2.0American Kestrel 2.4Ring-necked Pheasant 3.2European Starling 4.3American Goldfinch 4.9Speed of FlightIn estimating the flight speed of birds, scientists agree that there is no way of knowing a bird╒s true airspeed if a wind is blowing, unless one knows the speed of the air mass (wind) with which the bird is being carried, or against which it is flying. But what interests most of us is how fast a bird gets over the ground regardless of the force or direction of the wind.Some birds fly much faster in their courtship flights, some when pursuing their prey, and some when being chased by a predator. In other cases--flocks of migrating shorebirds and swallows, for example--each bird may fly faster when in the flock than when flying alone. Birds have a reserve of speed that, when needed, can carry them a third to twice their normal, or ╥cruising,╙ speed. The Peregrine Falcon, for example, may fly in ordinary flight at 40-60 m.p.h. but is capable of attaining at least 175 m.p.h.--the airspeed of the observer╒s plane when the falcon passed it--in its drop, or dive, in pursuit of prey. Six racing pigeons (Rock Doves, which the Peregrine Falcon often captures in flight) have been timed at 28, 52, 60, 65, and 82 m.p.h., showing the great variation in flight speed of which the species is capable.A male Red-breasted Merganser, pursued by an airplane over Alaska, attained a top airspeed of 80 m.p.h. along a river for 1,500 feet before turning aside. With a tailwind of 20 m.p.h. assisting it, the bird apparently achieved a ground speed of 100 m.p.h. Dunlins, godwits, and curlews have been clocked at 110 m.p.h. by an observer in an airplane overtaken by the birds.One of the more accurate ways of measuring the flight speeds of birds is with radar units similar to those used by police to detect speeding motorists. Measured flight speeds include (in m.p.h.):crows 31-45small perchers (e.g., larks, buntings) 20-37starlings 38-49geese 42-45ducks 44-59House Sparrows are among the slowest birds, with measured speeds at 16-19 m.p.h. Apparently the minimal speed birds must maintain in order to remain airborne is about 16 1/2 feet per second, or about 11 m.p.h.
