Warm-blooded organisms capable of flight.
About 8,800 species of birds make up most living organisms capable of flight. Believed to be evolved from reptiles, their weights vary from a few ounces, for tiny, flying hummingbirds, to to 300 pounds, for flightless ostriches. Most birds, however, fly well, and humans learned a lot about heavier-than-air vehicle design from observing them. Birds differ from heavier-than-air aircraft primarily in that their wings are movable, or flappable. Most aircraft have fixed wings, which do not move.
Birds’ bodies are specially engineered for flight. Their skeletons are light, often weighing less than their feathers. Feathers combine the qualities of lightness, strength, and flexibility; a feather, bent double, quickly regains its shape upon release. Made of keratin, feathers also keep birds warm, dry, and protected from injury.
Bird lungs and hearts are designed for the high metabolic rates needed to produce the huge amounts of energy required by all flying machines, biologic or manufactured. Birds’ respiratory systems allow for a much larger oxygen uptake than that of earthbound animals. Birds also have relatively large hearts, capable of passing all the oxygen needed for energy metabolism through the circulatory system to the other tissues.
To understand flight requirements, a background in aerodynamics, a branch of fluid dynamics that studies movement of bodies, such as birds or aircraft, through gases such as air, is essential. For example, the fifteenth century Italian artist and engineer Leonardo da Vinci studied bird flight and proposed to enable human beings to fly with flappable wings. His ideas failed because da Vinci knew nothing about aerodynamics, a science which did not exist then.
Any heavier-than-air flying vehicle must conquer gravity before it can climb into the air in controlled flight. Three main forces, exclusive of weight, are involved. The first is thrust, which birds produce by flapping their wings. Flapping merely enables a bird to move forward as long as its design allows enough thrust to exceed the drag caused by the viscosity of the air through which the bird moves. Drag diminishes the speed of moving objects due to air resistance. In vehicle design, thrust-to-drag ratios can be increased by streamlining to minimize drag.
The third aerodynamic force, lift, is the key to flight. Lift, enabling an object’s rise into the air, operates upward perpendicular to the direction of forward motion, and is supplied in both birds and aircraft by wings and tails (airfoils). Bird wings are designed so the angle at which they meet air passing them causes it to flow much more rapidly past the upper airfoil surface than past its lower surface. This design lowers air pressure above the airfoil compared to that under it and engenders the lift that raises a bird into flight. In birds, this unsymmetrical airflow is produced by muscle movement that changes both the positions of wing feathers and the angle at which wings meet the air, known as the angle of attack.
The importance of the angle of attack is demonstrated in aircraft by a pilot’s use or misuse of the angle during flight. An aircraft’s angle of attack is changed by altering its position in space. Angles of attack of up to 15 degrees increase lift and enable faster climb rates while also slowing airspeed. If the angle is too steep, decreased lift occurs, making the aircraft drop toward the ground, or stall. When pilot misjudgment causes a stall, an aircraft will crash unless the angle of attack is adjusted to a safe value. Birds, unlike aircraft, constantly make quick wing adjustments, moving their wing muscles to prevent stalls.
Birds create lift with downstrokes of their wings, attached by flight muscles to a large breastbone. Birds contract flight muscles to cause this downstroke, during which long primary and secondary flight feathers spread out to provide the maximum possible surface area to push against air below. The downstroke is followed by an upstroke in which the feathers fold to minimize air resistance while positioning the wings for the next downstroke.
Bird wings have a short upper arm bone that moves up and down during flapping. They also have rigid elbow and wrist joints that move horizontally to spread or fold the wing. Furthermore, the wrist and hand bones are a carpometacarpus, derived from palm bones, a one-boned thumb, a two-boned second finger, and a one-boned third finger. Flight feathers are attached to wing bones. The primary flight feathers, most essential to flight, attach to the carpometacarpus, second finger, and third finger. Up to forty somewhat less important secondary flight feathers attach to the ulna, one of the forearm bones.
When a bird opens its wings, the bones straighten, the primary feathers spread as the elbow joint extends, and the wrist stretches. Wingspread is limited by a tendon running from shoulder to wrist. The bases of the flight feathers interconnect via a ligament running from the elbow to the second fingertip. Spreading a wing stretches the ligament, moving flight feathers into positions perpendicular to the bones to which they are attached. While the wing is spread, muscle action can either spread primary feathers further or fold them back. The greater importance of primary feathers is clear, because removing even their tips prevents flying, while more than one-half of each secondary feather must be removed to do this.
There are four basic types of bird flight. In skimming flight, birds such as albatrosses use winds to stay aloft. In soaring flight, birds such as eagles, hawks, and vultures can remain aloft for long periods of time, seeking prey below. In active flight, birds such as swallows fly all day, flapping their wings continuously. Finally, game birds such as quail conceal themselves and, when endangered, burst into the sky. They pick up speed quickly and fly short distances before landing and hiding again.
There is a wing shape most efficient for each flight type. Skimming birds have wings that are long, slender, and ribbon-shaped, with parallel edges and many secondary feathers. Skimming wings are the most highly developed, helping such birds ride the winds. Soaring birds have wings that are large, broad, almost square, and rich in primary feathers. Swallows and other birds engaging in active flight have long, tapering, pointy wings with broad bases and slender tips. Finally, game birds have short wings that beat rapidly, enabling them to get to speed quickly. However, these wings are not useful in long flights.
No bird has wings designed entirely for one type of flying. However, in gliding, birds use gravity as thrust to overcome drag and move forward, as their wings produce lift to hold them up. Drag slows down a gliding bird and causes it to sink earthward. To maximize glide time, or soar, a glider sets its wings at the angle of attack giving a good lift-to-drag ratio. Low forward speed helps, and, to alter speed, such a bird spreads its wings to increase their area and reduce glide speed or closes them to cause the opposite effect. Long-winged birds glide by adopting positions with small glide angles, avoiding stall by twisting the wings to reduce the angle of attack. This angle can also be varied along the length of each wing. For example, gliding birds may have their secondary feathers at a high angle of attack and their primary feathers flat.
Active flight requires thrust force and expenditure of energy sufficient to overcome drag and keep the bird on course. This is achieved by flapping the wings for lift and propulsion. The wing parts function differently at each stage of a wing beat. For example, many fast-flying birds start downstrokes with wings fully extended and well above the horizontal. As they flap down vertically, forward movement through the air generates lift along the entire wing. At a downstroke end the wings fold and primary feathers close. No propulsion is generated in the upstroke, at the end of which the primary feathers produce enough lift to raise and extend the wing, preparing for the next downstroke.
A second group of characteristics enabling bird flight is the design of the bird’s body. Body weight is important to flight: The heavier an object is, the larger its wings need to be to enable liftoff and maintain flight. In birds this problem is met by their relatively small, light bodies. For example, hawks and eagles have cat- or even dog-sized bodies, but they weigh only 25 to 35 percent as much as the earthbound mammals. Birds’ light weight is due to several factors. First, under their feathers, birds have relatively small bodies. Second, although their feathers are bulky, they are also very light. In addition, birds have fewer bones compared with other animals, and their bones are thinner, or even hollow. This special anatomy, combined with wings that engender appropriate amounts of lift, allows birds to fly. Depending on their wing size and shape, birds can fly, soar, or skim.
To meet the energy needs of flight, birds must eat a relatively large amount of food each day. For their muscles to work well, birds need efficient blood circulation to quickly supply fuel and oxygen and to remove wastes. In both birds and mammals, the blood circulatory system has a four-chambered heart that directs blood to the lungs, where the blood picks up oxygen and then travels on to the muscles and other organs, where the oxygen is used. Carbon dioxide is picked up at the same time and carried, via blood, to the lungs for disposal. The difference between bird and mammal circulatory systems is the relatively larger size and greater power of a bird heart, which is two to three times heavier, compared to body weight, than that of a mammal. Bird heartbeat rates are also much faster than those of mammals, usually from 200 to 1,000 beats per minute, compared to 80 in humans. The combination of a large heart and a faster pulse rate results in a blood-pumping capacity for birds that is relatively much greater than that of mammals.
A bird’s respiratory system is very different from the bellows-type lungs of mammals. Bird lungs are relatively small, but they connect to inflatable air sacs located throughout the body, even in bones and breast muscles. These sacs are thought to cause very efficient exchange of oxygen and carbon dioxide with the bloodstream via one-way airflow through the lungs. When a bird inhales, air enters the lungs, posterior air sacs, and anterior air sacs. Exhalation causes air from posterior sacs to enter the lungs, and air from anterior air sacs is exhaled. Thus the air constantly passes through the lungs, ensuring a more efficient absorption of oxygen and removal of carbon dioxide compared to that of mammal lungs, in which only a fraction of the air is flushed out at each breath. Bird lungs are not worked via diaphragm. The air sacs are pumped by rib movement.
Thus, with its wings; its small, light body; its superbly useful feathers; and its high-capacity heart and lungs, a bird is superbly designed to be airborne.
Allen, John E. Aerodynamics: The Science of Air in Motion. New York: McGraw-Hill, 1982. Discusses aerodynamic principles, including some history. Its text and diagrams clarify many issues important to understanding lift, drag, and other issues essential to understanding heavier-than-air flight. Brooks, Bruce. On the Wing: The Life of Birds from Feathers to Flight. New York: Charles Scribner’s Sons, 1989. Discusses aspects of bird life, including feathers, eating without teeth, and flight. Chatterjee, Sankar. The Rise of Birds: 225 Million Years of Evolution. Baltimore: Johns Hopkins University Press, 1997. Describes the evolution of birds, the fossil remains of their ancestors, and avian flight. Freethy, Ron. How Birds Work: A Guide to Bird Biology. Poole, Dorset: Blandford Press, 1982. Thoroughly addresses the biology of birds, including their flight. Harrison, Colin, and Howard Loxton. The Bird: Master of Flight. London, England: Blandford Press, 1993. Covers avian flight completely.
Evolution of animal flight
Forces of flight