Feathers and flight birds

The two factors that set birds as a class apart from nearly all other vertebrates are feathers and flight. Only bats, which are mammals, are also capable of powered flight. Feathers provide insulation and the lifting surface of the wings. The mobility provided by flight has enabled birds to spread and establish themselves in nearly every habitat on earth.

Feathers

Bird feathers are made of the protein keratin, the same basic material as their horny bills and the scales on their legs. Each feather develops from a knob (papilla) within a feather socket, or follicle. The follicles are arranged in distinct areas called feather tracts over the bird’s body. Each follicle produces one, two, or even three feathers every year.

There are two main types of feathers pennae and plumulae. The pennae are the flight and contour feathers. A typical penna has a broad, flat vane attached to both sides of a long central shaft. The shaft consists of two parts. A hollow, rounded base, called the calamus or quill, extends from the bird’s skin to the vane. The solid, tapering upper part of the shaft, called the rachis, runs through the vane. The vane is formed by barbs that branch out from the sides of the rachis and barbules that branch from the barbs. Hooks on the barbules link neighboring barbs, giving the vane both strength and flexibility.

The flight feathers (primaries and secondaries) are directly concerned with flight, whereas the contour feathers have several functions. They give the bird its streamlined shape, helping it to cut through the air efficiently. They also provide vital insulation by trapping a layer of heat-retaining air close to the skin. During hot weather, a bird opens up its contour feathers to allow heat to escape, and in cold weather it fluffs them up to increase the insulating layer of air.

Another function of feathers is to waterproof the bird. They give the bird its color and may be used for camouflage, for species recognition, for sexual display, or as warninq signals.
The plumulae are the down feathers. They lie beneath the contour feathers, providing extra insulation, and are the only feathers on a newly hatched chick. Down feathers are much simpler than pennae, with a very short midrib and no barbules, so that the barbs are separate, giving them their fluffiness.

The number of feathers varies considerably from one species to another. Usually, the larger the bird, the more feathers it has. For example, a tiny hummingbird may have fewer than 1,000 feathers, whereas a large swan may have more than 25,000. The number of feathers also varies from one season to another.

Because feathers are subject to great stress and can wear out, all birds molt at least once a year. Most birds molt their feathers gradually, so there is no interference with flight. But ducks lose their flight feathers all at once and so experience a period when they are flightless and particularly vulnerable.

Despite their proverbial lightness, all the feathers on a bird are, surprisingly, often heavierthan the bird’s incredibly light, hollowboned skeleton. The feathers of the bald eagle, for instance, weigh more than 1.5 pounds (670 grams), but the skeleton weighs only about 9.5 ounces (270 grams).

Bird flight is complex, but the main feature is the wingbeat The sequence (above, right) begins with an upstroke, in which the primary feathers open to allow air to flow through them (A to D), making it easier to lift the wing. On the downstroke (E to G), the primary feathers overlap each other to form a broad, closed surface, which helps power the flight through the air. A flight feather (right) has a central midrib (rachis), bearing a flattened vane made up of hundreds of interlocking barbs, each of which has tiny barbules that hook together. The calamus, located at the base of the rachis, is hollow, allowing for the passage of nutrients to the feather.

Flight

Bird flight is governed by the same laws of aerodynamics that control all heavier-than-air flight. There are two opposing forces involved in the mechanics of bird flight lift, the upward force that keeps the bird airborne, and drag, the force of the air opposing lift, which is largely a result of air turbulence at the wingtips.

A bird’s wing is rounded above and hollow below, with the leading edge thicker than the trailing edge—a shape that produces maximum lift. The convex upper surface causes the air flowing over it to travel faster than the air flowing past the undersurface. As a result, the pressure on the underside of the wing is greater than that on the upper surface, producing lift.

The angle that the wing makes with the horizontal is called the angle of attack. The greater the angle of attack, the more the airflow over the wing becomes turbulent and the more the bird is likely to stall. At the optimum angle for flight, upward lift counteracts the forces of gravity and drag and keeps the bird aloft.

The main function of the primary feathers is to provide forward thrust. The highly flexible primary feathers of the wingtips twist, acting almost like a propeller that drives the bird through the air. The wingtips move faster than the rest of the wing. The main function of the secondary feathers is to provide lift—they move very little as the wing beats. The frequency of wingbeats varies greatly, from about two beats per second in a swan to 50 or more per second in a hummingbird.

To alter the direction of flight, a bird increases the angle of attack of one wing, or beats that wing faster, creating a difference in lift on the two sides of its body. Movements of the tail are also used. To land as gently as possible and minimize the shock to its body, a bird must be on the point of stalling. To achieve this, it pushes its body downward and spreads its wings and tail.

Flight speeds of birds also vary, although typical speeds are between 20 and 35 miles (32 and 56 kilometers) per hour. The fastest bird is probably the peregrine falcon, which dives on its prey at speeds of 180 miles (290 kilometers) per hour or more.

Some birds save energy by gliding instead of flapping their wings. Vultures, for instance, use their long, broad wings to soar in thermals (rising currents of warm air). Albatrosses tack back and forth low over the waves with their long narrow wings, using the updrafts produced by the friction of the air with the water to stay aloft.