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Flight (Summer, 2000)
Air Borne Winging It Leatherwings

 

Built to Fly

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Birds have committed themselves to flight through great physical adaptations. Any pilot knows weight is an important consideration in flying. Birds have jettisoned extra weight from their design. Their slender bones are hollow and filled with air, but rigid and strong too. Some of the bones, like those of the pelvis, have fused to strengthen and simplify the skeleton. The breastbone has a deep keel that allows the attachment of large flight muscles.

Feathers are finely engineered tools, designed to meet the flight needs of each species' lifestyle-stiff ones for rapid flight and maneuverability, soft ones for silent hunting. Even the respiratory sys-tem of birds has evolved to he much more efficient than ours. Flying is strenuous, with a high oxygen demand. Birds have a complex respiratory tract, involving air sacs as well as lungs, that allows more air to pass through the system than the simple in-and-out breathing of humans.

We might think of a bird as a flying craft with a two-part wing. The "upper arm" moves at the shoulder joint, providing lift through flapping. Seen in cross-section, the upper surface of the wing is rounded and the bottom surface flat. Air flowing up and over the curved top moves faster than the air flowing under the wing (you might think of it as having to "catch up" with the air underneath). This difference in air speed causes lower air pressure on the upper side of the wing, which in turn creates lift. The angle of the wing controls the amount of lift, thus a bird coming in to land slows down by tilting its wings to reduce lift and create drag. A few inches from the ground, the bird tilts its wings into a stall, then lands gently (hopefully). 

You won't hear the whir of wings from a flying great horned owl. The leading edge of the owl's primary feather has a "saw-tooth" design that breaks up air passing over the wing and eliminates noise. Wide and rounded, the soft secondary feathers also help the owl to move through the air silently.
	great horned owl feather

The outer half of the wing, the "hand," is flexible and acts like a propeller. A bird is able to spread those outer flight feathers, called primaries, like spreading the fingers of its hand. The primaries' flexibility allows the bird great control during flight. Watch a bird flying or hovering in the wind. You can see the many adjustments of individual feathers it makes to control its flight.

It was long thought that birds rowed the air with their wings, pushing back and down as they flapped. But a French scientist discovered in the 1860s—by sticking white paper on the wingtips of crows, then photographing them in flight-that on the powerful downbeat, birds sweep their wings down and forward. At the bottom of the stroke, the wrist flexes up, the "fingers" spread and the wing beats upward with the primaries vertical to reduce drag. Thus it is the downstroke that powers flight and propels the bird forward.

The power behind bird flight comes from two major pectoral muscles. Like lumberjacks on either end of a giant saw, the strong depressor muscle and the weaker elevator muscle work in concert to move the wing up and down. Anchored at the breastbone and the upper arm bone, the depressor muscle contracts and pulls the wing down. When it relaxes, the elevator muscle pulls, the wing back up.

Hummingbirds are the odd birds in this arrangement. Their wings are "all hand," and the wing moves entirely from the shoulder, providing them exceptional maneuverability. Watch a hummingbird alighting at a feeder. It lays its stiff wings on its back in one motion, without the folding and tucking done by other birds. This also allows hummingbirds to take off in a flash since they only need to lift their wings and begin flapping without unfolding them first. Of course hummingbirds are unique in their flying capabilities. They can fly backward-intentionally and under control-hover, and even fly upside down briefly. To achieve this aerobatic excellence, hummingbirds flap their wings 50 to 60 times a second.

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The ability to fly offers tremendous advantages, but if its delicate dynamics are changed, things can get tricky for the bird. A hunting osprey flies over a lake, spots a fish, plunges to the water's surface and grabs the fish with its talons. In retrieving its prey, the osprey compromises its flight dynamic. Now it is at a complete stop with part of its body in the water.

	American kestrel feather
The pointed, stiff flight feathers of the American kestrel cut the air, allowing it to fly swiftly and change direction quickly. These small falcons often hover while hunting, a maneuver that would be difficult or impossible for a soft-winged owl.

To become airborne again, the osprey must expend a great amount of energy. Only through vigorous flapping can it gain enough lift to break away both from gravity and the drag of the water. The bird also carries the added weight of the fish. With no surface to push off from and little help from wind or air movement, the osprey relies on muscle strength to essentially pull itself from the water into the air.

The heavier a bird is, the faster it must fly to stay airborne. Have you noticed how ducks flying overhead flap their wings very fast? Compare that to the lazy soar of a turkey vulture. The heavy-bodied ducks have a small wing surface for their weight. By contrast, the vulture has large, broad wings for its body size. Its weight is distributed over a greater wing surface area, meaning it has light wing loading. If a duck stopped flapping and tried to soar, it would fall out of the air like a lead weight. British scientist Sir D'Arcy Thompson calculated that the flightless ostrich, weighing 250 pounds, would need to fly at 100 miles per hour or more to stay in the air.

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So how fast can birds fly? Peregrine falcons are the Olympic sprinters of birds, capable of 40 to 60 miles per hour during ordinary flight, but able to plummet during their predatory stoop at up to 200 miles per hour. This is not a freefall but a controlled drop (headfirst), with the partially folded wings held out. After striking (or missing), its prey, the peregrine immediately pulls out of the stoop and flies off either to retrieve its dinner or to try again. Racing pigeons, which are often the prey of falcons, have been clocked at speeds of from 28 to 82 miles per hour. A California man, who clocked the speeds of birds in his car as they flew alongside a highway reported a flicker flying at 44 mph, a Savannah sparrow at 37 mph, and a cinnamon teal at 32 mph, which sped up to 59 when he pressed it. It's not reported whether the teal got a ticket.

Have you ever watched a turkey vulture soar on summer thermals, higher and higher until it was no larger than a speck of pepper? Or seen the sparkle of white shapes in the cloud and realized they were a platoon of high-flying pelicans? While the small birds in your back yard seldom fly higher than 50 to 100 feet, vultures and pelicans may soar to 5,000 feet. The super high altitude records for bird flight come from mountain ranges like the Himalayas and Alps—obstacles migrating birds must fly up and over. Bar-headed geese were recorded migrating over the Himalayas at more than 27,000 feet and a Ruppell's griffon, a species of vulture, was struck by a commercial airliner over West Africa at 37,000 feet. In North America, the highest flying record is of a mallard that hit a plane at 21,000 feet.

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Why do Canada geese fly in a "V"? It has nothing to do with their ability to spell. As a bird flies, some of the lift produced is lost over the tips of the wings. The moving air tumbles off the wingtips, creating an up-spiraling vortex. By flying just off the wing of the bird in front, a goose can make use of its neighbor's lost lift and save energy. Researchers estimate that birds in formation can fly 71 percent further, on the same amount of invested energy than a goose flying alone.

The Gambel's quail has stiff feathers to power its explosive flight as it flushes from hiding. The sudden noise and whirring made by the feathers also aid the quail's escape by startling and confusing predators.
Gambel's quail feather

What image of the West is complete without a red-tailed hawk soaring lazily on summer thermals? Soaring birds make use of rising columns of warm air to gain lift without flapping. They can maintain altitude or ride the columns higher and higher. To soar, a bird must have large, wide wings that give it light wing loading. Small birds can't soar very well because they are too easily tossed around by air turbulence. The classic soaring birds are the vultures, condors and buteo hawks, such as the red-tailed, Swainson's, ferruginous and rough-legged. To keep within these patterns of rising air, or to take advantage of shifting conditions, soarers must be able to make tight turns, change direction quickly or drop down to capture an air current. 

Like soaring, gliding flight occurs without flapping. A gliding bird is in a descending trajectory, coasting down-ward against the air resistance. To regain altitude, it must begin flapping or find a rising thermal. Some birds ride the air for hours, soaring up and around, then gliding down to soar once again upward. To tell whether a bird is gliding or soaring, check its wing profile. Primary feathers are held out and slightly forward during soaring, cocked back-ward when gliding.

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Courtship flight brings out some of the fanciest flying in the bird world. Courting bald eagles fly close together, then the male flips onto his back beneath the female, the two clasp talons, and tumble through the air. A male harrier performs a sky dance for the perched female, flying straight up in the air, then plummeting toward, the ground before pulling up in a steep "U". Nighthawks do a similar dive, control-ling the passage of air through their wing feathers to produce a loud szhoomp at the bottom of the dive. The lark bunting flies up high, then floats downward in a spiral on stiff wings like a butterfly.

Swifts are true masters of the wing; eating, drinking, sleeping, bathing and even mating while in flight. Only nesting requires them to come to earth, and even then they choose high ledges and crevices. A swift's feet and legs are so small and weak they would have difficulty taking off if they did land on the ground. Their family name, Apodidae, appropriately means "footless." The swift's long, slender wings curve backwards and are designed for high speed flight. The pointed wing shape and stiff feathers allow the wing to cut the air during sharp turns and maneuvers.

Next: Winging It

(The information contained in this issue of Colorado's Wildlife Company was accurate at the time of original publication. Situations and circumstances described, staff positions, contact information, and dates of some events may have changed in the interim. Present knowledge and understanding of biological and behavioral facts and information may also be different, now, than presented here.)