FLIGHT ADAPTATIONS IN BIRDS
(Dr. Girish Chandra)
Flying is a balance between two sets of forces, lift and weight, and thrust and drag. Weight is the result of gravity and Lift is generated by the flow of air over the wings. Bird wings are not flat but are concave below and convex above. The air that passes over the top of the wing has more distance to travel and thus it speeds up, causing the pressure to drop because the same amount of air is exerting its pressure over a greater area above the wing that below the wing. This effectively sucks the wing up. Meanwhile the air going below the wing has the opposite effect. It slows down, generates more pressure and effectively pushes the wing up. Hence a bird with air moving over its wings is pulled up from above and pushed up from below. The low pressure of air on top of the wings represents a sink that the high pressure air under the wing tries to fill. This happens most along the thin trailing edges of the wing and causes a spiralling vortex of disturbance at the wing tip, which increase drag. Therefore, the most efficient wings are those which provide lift but reduce drag, such as the crescent shaped wings of swallows and swifts.
Aerodynamic properties are measured by aspect ratio, which is the ratio of wing length divided by wing breadth. Long wings are better for gliding but harder to flap quickly and are therefore not much good at quick acceleration. Wing loading is the relationship between total body-mass measured in grams versus total wing area measured in square centimetres.
Non-flapping Flight or Gliding
Many soaring or gliding birds like vultures hang in the air and gain height without moving the wings. Essentially this means that their wings generate a lot of lift without producing much drag. Large birds have evolved to be gliders partly because gliding becomes easier with larger wings and the mechanical flapping flight become harder with larger wings. With the exception of Hummingbirds, all birds glide to some extent when flying. As a rule, the smaller the bird, the shorter the distance it can glide and the faster it sinks. Gliding can be observed in game birds. A pheasant ascends from the ground like a rocket with fast wing beats and then glides for some distance down to the nearby woods.
Flapping flight is a more complicated process in which bird’s wing changes shape and angle of attack during both the up and down stroke. Flapping flight is basically rowing in the air with the added effort to generate lift as well. If a Blue Tit stops flapping its wings it better be about to land on a branch or it will fall to the ground. Flapping flight consists of two distinct movements: the power stroke and the back stroke. In the power stroke, the wings move forward and down; the back stroke returns the wings to the position from which the next power stroke will commence.
Soaring differs from gliding flight in that the bird does nor lose altitude and sometimes even climbs up in air. When soaring, a bird uses no energy of its own; instead it depends on external forces called thermal currents, which are rising masses of air that form over areas where the ground warms up rapidly. Obstruction currents are produced when wind currents are deflected by mountains, cliffs, or tall buildings. The resulting upward rise of air lifts birds to high altitudes, providing a base for further gliding. Soaring birds always have large and broad wings, and the ratio of their body weight to the size of the airfoils is low.
In hovering flight, a bird generates its own lift by means of rapid wing beats. Holding its body nearly vertical, with its wings firmly flexed at the elbow joint, a hovering bird moves its wing surfaces forward and back in a horizontal plane; each of the two phases of the stroke generates lift. Hummingbirds’ wings are made in such a way that when in motion they act like lifting rotors. Their pointed wings do not flap and glide as other bird wings do, but propel them through the air by moving up and down, at a rate of 70 times a second. After feeding at a flower they can fly backward, climb vertically, turn at lightening speed, and come to a sudden standstill in midair. Hummingbirds have been known to fly up to speeds of 60 miles an hour and no bird of prey can ever catch a hummingbird in flight.
Birds have short, light and compact body as compared to other animals.
Most organs and large muscles are located near the center of gravity, which is slightly below and behind the wings to provide better balance during flight.
Contour feathers cover the body and make it streamlined and decrease drag. Down feathers and soft and meant for insulation. Primary feathers are on the wings and are also called remiges, which help in flight and also provide wing shape. Tail feathers are called rectrices which stretch sideways so that tail can be used like a rudder for turning and balancing.
The evolution of flight has endowed birds with many physical features in addition to wings and feathers. One way to reduce weight in birds is by the fusion and elimination of some unnecessary bones and the “pneumatization” of the remaining ones. Not only are some bones of birds hollow but many of the larger ones are connected to the air sacs of the respiratory system. To keep the cylindrical walls of a bird’s major wing bones from buckling, the bones have internal strut-like reinforcements. Fusion of bones in birds makes the skeleton light as well as strong. Coracoid, furcula, and scapula form a sturdy tripod for supporting the wings and broad surfaces for the attachment of large flight muscles.
One key adaptation is the fusing of caudal bones into single pygostyle which supports the tail feathers. Birds also lack teeth or even a true jaw, instead having evolved a beak, which is far more lightweight.
Birds have uncinate processes on the ribs. These are hooked extensions of bone which help to strengthen the rib cage by overlapping with the rib behind them.
Skull is composed of thin, hollow bones,which is extremely light in proportion to the rest of the body due to elimination of a heavy jaw, jaw muscles, and teeth. The job of chewing has largely replaced by the gizzard. The skull usually represents less than 1 percent of a bird’s total body weight.
Forelimbs (wings) are attached closer to center of gravity and farther from head than in other animals. The natural motion of wings is up and down, rather than back and forth. Forelimbs fold into a Z-shape and closer to the body. Hand bones are small, fused, flattened and specialized to manipulate the flight feathers. Aerodynamic shape of forelimb provides lift and propulsive force.
There are about 175 different muscles in the bird. They mainly control the wings, the tail, neck and the legs. The largest muscles in the bird are the muscles that control the wings. They are called the pectorals or the breast muscles, and make up about 15 – 25% of a bird’s full body weight. They make the birds’ wing stroke very powerful so that they can fly, and provide most of the movements the bird needs for its down stroke. The muscle below the pectorals is the supracoracoideus. It raises the wing when a bird is flying. The supracoracoideus and the pectorals together make up about 25 – 35% of the birds’ full body weight. Leg muscles are massive for bipedal locomotion but are tight and close to the body. Legs are tucked next to body in flight to reduce drag. Flight muscles are enormous as they have to generate thrust and vigorous movement of wings during flight.
Birds consume high-energy foods such as insects, seeds, fruits, meat, and nec- tar. The digestive system is extremely efficient in absorbing energy from small amounts of food at a rapid rate. Birds possess a gizzard that is composed of four muscular bands that act to rotate and crush food by shifting the food from one area to the next within the gizzard. Depending on the species, the gizzard may contain small pieces of grit or stone that the bird has swallowed to aid in the grinding process. Many birds possess a muscular pouch along the esophagus called a crop. The crop functions to both soften food and regulate its flow through the system by storing it temporarily.
The respiratory system of birds is adapted to the energy demands of flight. A bird’s respiratory system is proportionately larger and much more efficient than in other animals, since flight is a more demanding activity than walking or running. An average bird’s respiratory system occupies about one-fifth of its body volume, while in an average mammal it is only about one-twentieth. Lungs of birds are less flexible, and relatively small, but they are interconnected with a system of large, thin-walled air sacs in the front and in the posterior portions of body. These, in turn, are connected with the air spaces in the bones. Inhaled air passes first into the posterior air sacs and then, on exhalation into the lungs. When a second breath is inhaled into the posterior sacs, the air from the first breath moves from lungs into the anterior air sacs. When the second exhalation occurs, the air from the first breath moves from the anterior air sacs out of the lungs, while the inhaled air moves into the lungs. The air thus moves in one direction through the lungs. All birds have this one-way air flow system and many also have two-way flow system which may make up as much as 20 percent of the lung volume. In both systems, the air is funneled down into air capillaries carrying oxygen-poor venous blood. At the beginning of the tubules the oxygen-rich air is in close contact with that oxygen-hungry blood, while in distal tubules the oxygen content of air and blood are in equilibrium. Birds’ respiration creates a “crosscurrent circulation” of air and blood, which provides greater capacity for the exchange of oxygen and carbon dioxide across the permeable respiratory membrane.
Oxygen exchange occurs during both inhalation and exhalation. The posterior and anterior air sacs expand during inhalation. Air enters lungs via trachea. Half of the inhaled air enters the posterior air sacs, while the other half passes through the lungs and into the anterior air sacs. The sacs contract during exhalation. The anterior air sacs empty directly into the trachea, the posterior air sacs empty via the lungs into the trachea and to outside.
Since during inhalation and exhalation fresh air flows through the lungs in only one direction, there is no mixing of oxygen rich air and carbon dioxide rich air within the lungs as happens in mammals. Thus the partial pressure of oxygen in a bird’s lungs is the same as in the environment.
Avian lungs do not have alveoli, as mammalian lungs do, but instead contain millions of tiny passages known as parabronchi, connected to air capillaries, where oxygen and carbon dioxide are exchanged with cross-flowing blood capillaries by diffusion. A diaphragm is absent in birds and instead the entire body cavity and air sacs act as bellows to move air through the lungs.
Bird’s heart is large, powerful, four-chambered and of the same basic design as that of a mammal. The segregation of the two kinds of blood makes a bird’s circulatory system, like its respiratory system, well equipped to handle the rigors of flight.
The flight muscles of most birds are red in color because of the presence of myoglobin and cytochrome. They are also richly supplied with blood and are designed for sustained flight. Light-colored muscles are found in pheasants, grouse, quail, and other galliformes birds. These are also well supplied with blood, are apparently capable of carrying a heavy work load for a short time, but fatigue more rapidly. A Ruby-throated Hummingbird’s heart beats up to a rate of 1200 beats per minute (about 20 beats per second). The human heart weight amounts to 0.42 percent of body weight and the pulse rate at rest averages 72 beats per minute. The House Sparrow’s heart constitutes 1.68 percent of the body weight and the pulse rate at rest averages 460 beats per minute. In the Ruby-throated Hummingbird these figures rise to 2.37 percent and a pulse rate of 615.
Blood of birds has high blood pressure and high blood sugar, almost twice that of mammalian glucose levels). Fast flying and migratory birds have smaller red blood cells with greater surface-to-volume ratios for greater oxygen-absorbing capability.
Brain is large with enormous cerebral hemispheres but the surface is white and without grey matter. Olfactory lobes are greatly reduced but optic lobes are excessively enlarged. Cerebellum is well developed with a median lobe, vermis and later lobes flocculi for coordination of muscular activity and balance.
Eyes are large, with wide field of view and binocular vision. Nictitating membrane is transparent or translucent and covers the eye ball during flight. Sclerotic ring of bony plates protects the eye ball and increases the distance between the lens and retina for sharp distant vision. Birds have acute eyesight, with raptors having vision eight times sharper than humans. This is because of high density of photoreceptor cells retina (up to 1,000,000 per square mm in Buteos, against 200,000 in humans). An indented fovea on retina magnifies the central part of the visual field. Many species, including hummingbirds and albatrosses, have two foveas in each eye, and the ability to detect polarized light is also common in birds.
Birds have high metabolism and endothermy for quick generation of power and for maintenance of high body temperature. Birds require large amounts of energy for flight, and need efficient oxygen circulation in high altitudes. The highest flight recorded for a bird was 11,274 m (37,000 ft.) when a Ruppell’s griffon vulture collided into a commercial airline over western Africa (Martin, 1987).
Birds normally maintain a body temperature of 38.0C to 42.0C (100.40F-107.60F) (Brooke and Birkhead, 1991).
Ovaries and testes are reduced in size except in the breeding season. Usually only one functional ovary is present in most of the birds and second ovary is greatly reduced to decrease the weight of body. Female liver is displaced to the right to compensate for weight difference. But in the case of birds of prey generally both ovaries and oviducts are present. This is because during hunting these birds have to pounce on the prey with great force and struggling prey can kick and break the eggs in reproductive system. Eggs developing in two ovaries can compensate for this loss.