"DOTH THE HAWK fly by thy wisdom, and stretch her wings to ward the south?" (Job 39:26).
Solomon listed as one of the things that were too wonderful for his comprehension "the way of an eagle in the air" (Prov. 30:19). The words of the old song, inspired by a psalm of David, "Oh that I had wings like a dove! for then would I fly away," reflect the emotions of men toward the feathered creatures.
An ancient Greek myth depicts a knowledgeable Athenian man, Daedalus, and his son, Icarus, both of whom fell out of favor with the king of Crete, and were exiled on a small island in the Mediterranean Sea. They naturally sought some means of escape. Daedalus, after studying carefully the design of the wings of the sea birds, made two pairs of wings out of wax and feathers. These wings, the story goes, enabled them to escape. But the escape ended in disaster. Icarus, excited with his ability to fly, and against his father's warning, flew too near the sun. The poet Erasmus Darwin describes the incident in this way. "With melting wax and loosened strings, Sank hapless Icarus on unfaithful wings." 3,000 years after the setting of this myth that man learned how to fly safely, but not by attaching wings to his shoulders. Man learned that his body is not designed for flight in other ways besides the lack of wings. For example, the breast muscles that operate the shoulders and arms in man weigh less than 1 percent of his body weight, while those of some birds may be as much as 30 percent.
A well-known ornithologist, Dr. Joel Welty,1 points out that the first biological commandment is physiological constancy. This means that the great struggle in most animals' lives is to avoid change. He further emphasizes that since birds can fly across oceans, deserts, forests, and mountains, they have exceptional opportunities for preserving their internal, physiological stability. In other words, with flight ability, birds can seek the external conditions and foods they require to keep their internal functions operating efficiently and steadily.
Although birds do differ considerably in color, size, shape, and in their strength of flight, they do not deviate widely from good internal and external aerodynamic design. These unique features include intricately constructed feathers, powerful wings, light, hollow bones, rigidity of skeletal parts, a large, strong heart for the efficient circulation of warm blood, a remarkable respiratory system, and a digestive system with the ability for rapid and efficient absorption of energy from food materials.
Remarkable Skeletal Features
Birds' skeletons show many features that are suitable to flight. For example, although the Man-O-War bird has a wing span of about seven feet, its bones weigh only four or five ounces. Its plumage actually weighs more than its skeleton. In spite of the lightness of the bones, they are unusually flexible and strong, features that are essential to cope with the stresses and strains of maneuverable flight. A bird's skull is about 1/6 the weight of a mammal's skull of comparable size; the tail vertebrae and pelvic bones are fused into an extremely light cylindrical structure; the breastbone with its deep keel for the attachment of the powerful wing muscles ex tends backwards, giving support to the internal organs while in horizontal flight; the long bones of the legs and wings are hollow and are supported internally with trusses and struts for extra strength.
Penetrating the hollow bones, even the small toe bones, is a remarkable system of air sacs that are actually extensions of the respiratory system. These sacs give buoyancy both in air and water and increase the respiratory and cooling-surface area. This arrangement greatly facilitates the exchange of oxygen and carbon dioxide from the body tissues, but only one quarter of the air intake is used for respiratory purposes. The remaining three quarters serve to cool the. active tissues.
High Body Temperatures
To cope with the tremendous energy demanded for flight, birds are equipped with the highest body temperatures of any animal. Compared to man's 98-99 degrees Fahrenheit, a bird's temperature may be as high as 110 F. This high body temperature, along with an efficient digestive system and rapid circulation, is responsible for the fact that birds utilize an unusually high percentage of the food that they eat. In water birds that eat fish even the bones are digested, and the wastes when eliminated are quite liquid. Also contributing to body lightness is the fact that concentrated urine is passed directly from the kidneys to the exterior, avoiding need for a urinary bladder.
Someone has calculated that the Golden Plover is so efficient it can migrate thousands of miles across the ocean from Labrador to the central part of South America by losing only about two ounces of its body weight. If a small airplane, which normally consumes a gallon of gasoline in twenty miles, were as efficient, it would be able to fly about 160 miles on a gallon of fuel. 2
A means for rapid delivery of energy to the breast muscles is also necessary. In birds blood pressure is higher than in man, and blood sugar concentrations average twice that of mammals. Weak-flying birds such as domestic chickens have a relatively poor blood supply to breast muscles, as evident in the pale color of the flesh; but strong fliers have good circulation in these muscles, and as a result the muscle is dark red in color.
To provide the keen sight required for flight the optic lobes of the brain and the eyes of birds are relatively large. It has been determined that, at least in some of the hawks and other birds of prey, vision is eight to ten times more efficient than in humans. This, however, does not mean that the hawks have telescopic vision; but it does mean that the eye is constructed for greater resolving power. The most sensitive part of the retina of a hawk's eye, the foveae, contains up to 1.5 million rods and cones. Comparing the equivalent macular area of a man's eye we count only 200,000 visual cells. This gives a hawk a distinct advantage over man in the ability to distinguish detail.
Structure of Feathers
But perhaps the most remark able and unique feature of a bird is its feathers. No other animal has feathers. More than any other feature, feathers give birds greater efficiency in the air than is attained by other flying forms. Feathers are amazingly structured for protection against heat and cold, for fanning the air, and for streamlining the body. Every slight change in position of a feather during flight is designed to absorb energy from the air and use it effectively. It has been estimated that, for their weight, feathers are stronger than any man-made material.
A close look at the construction of a single wing feather under a microscope reveals a truly marvelous design. Compared to the relatively simple scutes of reptiles and scales of other vertebrates, a bird's feather is much more complex in both structure and function. It seems incredible that this remarkable difference could come about by mere chance.
Wallace 3 states that a single pigeon primary wing feather consists of more than a million parts. Along both sides of the stiff quill, or rachis, are grooves that bear filamentous extensions called barbs. These form the flat vanes of the feather. Each vane usually contains several hundred barbs that are held together by many very small barbules, which in turn bear flanges and tiny hooks called hamuli. These parts work together to form a zipper, which, when it becomes unzipped, can be zipped up again by preening. The next time you find a feather run your fingers over the vanes toward the thick end of the quill. The flanges and hooks will separate and the barbs will look ragged and tattered. Then zip the feather part together again by slipping the vanes between your fingers to ward the tip of the feather. Several trials may be necessary to make the feather look neat again. The closer we look at this intricate construction the more we become aware that the feather is designed to give strength and flexibility in flight.
Efficient prolonged flight can occur only when the wings are so structured that every slight change in air flow is automatically compensated. The flight feathers of the wing tip act like the propeller of an airplane and must be motile and variable in pitch to cope with the changing stresses that the air exerts upon them. The base part of the wing acts like the wing of a plane; and the secondary and tertiary wing feathers function as flaps.
Embedded in the skin near the quill of each flight feather are numerous nerve endings that convert the feathers into sensory receptors. These record the precise position of each feather, and through intimate connections with reflex arcs in the spinal cord, bring about continuous variations and fine adjustments of more than 12,000 tiny muscles that are attached to the base of the feathers.
Phenomenon of Flight
The phenomenon of flight be comes still more marvelous as we consider that the precise body position of the bird is recorded by the semicircular canals of the inner ear, which reports the changing conditions to the cerebellum of the brain. It is also interesting to note that the cerebellum in birds is better developed than in most other animals. Its larger size is a result of the accumulation of great numbers of sensory fibers that receive transmissions of muscle tensions from the body and wings. The assembled messages are converted into signals that produce split-second coordination of the body movements.
Attempts to make detailed observations of the motion of a flying bird meet with problems, for the form of the wing is constantly changing during its rotation cycle. Some of these changes are related to flexibility and motion of individual feathers and others to the internal muscular movements of the whole body, which are under the control of the semicircular canals of the bird's ear and the cerebellum. We can only describe in over simplified terms the obvious features of these motions.
In general, the larger the bird, the slower it flaps its wings a hummingbird vibrates its wings about fifty times per second while a heron only about two times. During slow flight in confined spaces the downstroke gives lift and some forward motion; but the upstroke, which consists of a quick backward flip with the wings partly folded, is surprisingly responsible for both lift and more forward thrust than the downstroke. The upstroke becomes relatively passive in full horizontal flight, and the total motion is less labored and involves less energy than for flight in confined spaces. Take-off and landing demand the greatest energy and coordination.
Gliding flight may be represented by the motion of a toboggan sliding downhill, except that the bird is sliding on air instead of slickened ground. The giant albatross, which may have a wing span up to twelve feet, uses this type of flight almost exclusively. At sea albatrosses may be observed to glide over the surface for hours without any noticeable wing flap ping. This capability depends on the shape of the wings and on the angle that the surface of the wings creates with the direction of gliding motion. A light bird with large wings glides slowly, while a heavy bird with small wings develops greater speed.
Sustained gliding flight is also dependent on the motion of the air. If the air is motionless, the glider must of necessity eventually come to rest on the surface, and its length of flight will depend on the height from which the glide began. Air motion over the troughs and crests of waves is responsible for sustaining the long flights of shear waters and albatrosses at sea. To illustrate: if a shearwater is gliding down an airslope losing vertical altitude at a rate of ten feet per second, and the air at the same time is rising from the surface of the waves at ten feet per second, the bird will glide along in level flight.
If the air is rising from the water surface faster than the bird is gliding downhill, the bird will gain altitude without effort to do so. When the wind speed equals the forward and downward speed of a gliding bird the ground speed of the bird will be zero, and it will appear to an ob server to stand still. Under these conditions a bird may be likened to a man walking down an up escalator at the same speed it moves upward. His progress in space is then zero.
Some birds seem to glide for sheer pleasure on windy days. A few years ago I watched some gulls hover in the breeze above the high point of a rocky island off the west coast of North America. At the high est point was a lighthouse capped with a slender spire. I was amused to see several gulls taking turns at circling, then hovering near, and holding the tip of the spire with their bills. When one lost the grip the next in line would move in to hang on.
Next I noticed some guillemots, which have short wings and normally do not glide, circling over the crest of the rocky peaks, laboring slowly against the stiff breezes, then turning to glide down the leeside like playful otters sliding on snow. The guillemots tobogganed on air.
Land birds such as buzzards, hawks, and eagles may be seen to soar in circles, gradually attaining great heights, then dive swiftly downward to start at the bottom of another spiral. Their upward soaring is the result of a downward glide at a rate slower than the rising air of thermal up-currents. They dive in adjacent down drafts until another thermal is encountered. According to detailed observations published by Cone 4 this is a highly complicated process one that the bird accomplishes by expending only a minimum of energy.
Everyone has at some time seen soaring birds with their wing tips spread like fingers on a hand. This function, called slotting, serves to prevent air turbulence behind the wings when the wings are tilted downward at the trailing edge. Slotting the wings is also necessary when a bird is coming in for a. landing and is quite characteristic of flight at reduced speeds.
The helicopter actions vertical, reverse, and hovering flights of swifts, hummingbirds, and skylarks are extremely complicated patterns of coordination and motion. The mechanism of these special types of flight is not fully understood, but it is recognized that these acrobats of the air use not only a powered downstroke but also a powered up stroke. In several characteristics a hummingbird's wings are quite like the wings of many insects. They operate backwards and forwards in a rowing motion, but they also tilt like the rotors of a helicopter and the wings of a dragonfly.
A bird in flight propelling itself by its own coordinated muscular efforts is surely one of Nature's masterpieces. The whole phenomenon of bird flight is so fantastically intricate that I can only conclude that its origin must lie, not in the realm of haphazard chance, but in intelligent design.
1. Joel Carl Welty, The Life of Birds (Philadelphia: W. B. Saunders Company, 1962), p. 1.
2. Ibid., p. 8.
3. George J. Wallace, Introduction to Ornithology, 2d edition (New York: The Macmillan Company, 1963), p. 44.
4. C. D. Cone, Jr., "Thermal Soaring of Birds," American Scientist 1962:50, pp. 180-209.