The Miracle of Bird Flight
By “Awake!” correspondent in Australia
A CROWD gathers at Stirling Castle in Scotland to witness a spectacular event. There on the roof is an Italian alchemist who had announced that he would fly to France using specially designed wings well endowed with bird feathers.
He is off! But where does he alight? At the foot of the castle, with a broken thigh bone! So ended a sixteenth-century attempt to imitate the miracle of bird flight.
People have always been fascinated by the flight of birds. About three thousand years ago a keen observer of the world around him stated that “the way of an eagle in the heavens” was just “too wonderful” for him to understand.—Prov. 30:18, 19.
Many persons long felt that if man simply possessed feathers like a bird and flapped wings up and down he could fly. However, over the last two centuries or so, man has come to realize that birds are more marvelously equipped for flight than previously thought. This unique equipment includes their feathers, wing shape, specialized muscles, body shape, bone structure and, of course, their instinctive ability to handle the many variable factors of flight. Man has learned much from the birds, and he has invented machines that can clumsily imitate their flight. But he is just not designed to fly as they are!
The Need to Fly
Man, of course, does not need to fly to survive, although most birds do. They are very active creatures, requiring large quantities of food. For example, their hearts beat between 200 and 1,000 times per minute, and they have a body temperature of between 102 and 112 degrees Fahrenheit (39 and 44 degrees Celsius). It has been estimated that the common swift may fly 12 to 14 hours daily at speeds of about 40 miles (65 kilometers) per hour in normal hunting flight. When feeding its young, this bird may cover about 600 miles (960 kilometers) a day.
For short periods, some birds can fly at tremendous speeds. Falcons have been known to dive at about 180 miles (290 kilometers) per hour. In India, spine-tailed swifts have been timed at about 200 miles (320 kilometers) per hour.
As an observer watches a bird fly so effortlessly, he cannot help but wonder: How do birds do it? How do they manage to stay up in the air?
The Secret of Flight
Although we cannot see the air around us, we know that, when it is in motion, it can be very powerful. In a storm, trees can be uprooted and roofs can be lifted off houses. Similarly, air moving around the specially shaped wings of a bird provides enough upward lift to counter the pull of gravity and prevent the creature from falling to the ground. Without the effect of moving air, a bird would drop to the ground like a stone.
A bird’s wing is so shaped that the air must travel farther over the top of the wing than underneath it. Hence, the air above the wing travels faster to ‘catch up,’ as it were.
Due to increased speed, the air above the wing is “thinner” than the air below. The compressed “thicker” air under the wing exerts greater pressure and pushes the bird up, providing the needed lift. Something similar occurs when you drink with a straw. As you suck on the straw, you are thinning out the air inside the straw. The normal air outside is then “thicker” and pushes the liquid up the straw.
The air striking the underside of the bird’s wing also tends to lift it. At the same time, however, the creature has to use some of its strength to overcome the air’s dragging effect.
To get airborne, a bird usually jumps into the air, flapping its wings. At first it may seem that the bird just flaps them up and down. But closer investigation reveals that this flapping flight is far more complex. The bird pulls its wings down and back with the feathers tightly closed and wings outstretched, thereby pushing as much air as possible. Then it pulls the wings forward and up with the feathers separated to allow the air to pass through. The wings are also pulled in close to the body so that there is minimum air resistance.
Wing movement provides lift as well as the propulsion needed to overcome “drag” and to gain speed. The bird’s wing movement might be compared to a swimmer’s doing the “butterfly stroke.” His arms rotate around his shoulder joint, as he throws them forward through the air and then pulls them back through the water. Flight, however, is far more complex, involving the rotation of the wing and the relative movements of various parts of it.
The faster the bird travels, the more lift will come from the air passing around the wings. It has been calculated that a pigeon first taking off uses five times as much energy as when it reaches steady flight.
With most larger birds, the increased wingspan is still not large enough to cope with their extra weight and greater drag, especially when taking off. So some of these, such as the pelican, run on the ground for a few feet to gain speed for lift. Others, such as the vultures, land on a tree or fence and then, by jumping off, gain enough speed through the pull of gravity for their wings to provide lift.
The heaviest bird that can fly is the trumpeter swan, weighing up to 40 pounds (18 kilograms). Heavy birds are limited in the amount of flapping they can do because of the strenuous effort involved. However, this does not restrict their ability to fly, for they are masters of another form of flight.
Soaring and Gliding
Large birds can fly for hours over great distances with hardly a movement of their wings due to their use of air currents. We might draw from common experience to illustrate what these currents are. Whenever you put your hand above something hot, you can feel the warm air rising. Similarly, when the sun heats the earth, some areas heat up more than others, depending on the nature of the surface. This causes the air above the surfaces to rise, producing a strong current of air, even though on the ground everything may appear quite still. These updrafts, called “thermals,” are doughnut-shaped bubbles of hot air and have been known to rise to heights of 10,000 feet (3 kilometers).
Another way an updraft is produced is when the wind strikes a hill or a mountain. The wind is forced up the mountainside, and this air movement continues past the mountaintop.
When a bird finds an updraft that is rising at a faster rate than the bird would be descending, it can “ride” on it, usually circling as it goes, to stay within the rising air. Like a sail, the outstretched wings catch the updraft. Thus birds can gain altitude with virtually no effort. This type of flight is called “soaring.”
Associated with this is “gliding” flight, in which the bird descends with its wings outstretched and all surfaces expanded to slow its descent. The best gliders can travel a distance of about 20 times the altitudes from which they start to descend.
Gliding birds, such as vultures, gulls, pelicans, hawks and eagles, can travel vast distances with very little effort by rising in one updraft and then gliding till they reach another. Through wing movements they can just hover at the same height in an updraft or change from a glide to a soar in an instant. Some bird varieties can travel this way at speeds of 30 to 50 miles (48 to 80 kilometers) per hour for most of the day, thereby conserving their energy. Usually one can tell when birds use this type of flight, for they will circle for a while as they rise and then change into a long, straight glide.
Birds such as the albatross are experts at handling the strong winds over the ocean. With the wind behind it, the albatross starts a long glide toward the water’s surface, gathering speed. A few feet from the water it turns into the wind and is lifted up by it, gaining altitude but losing speed. Then it turns and starts the cycle again. By adjusting the distances traveled in any part of this cycle, the bird can travel in any desired direction. By this technique, the royal albatross, for example, can travel at 50 to 70 miles (80 to 110 kilometers) per hour for long periods. The only effort required is that the bird must keep its wings extended and occasionally flap them once or twice.
Since flapping requires great amounts of energy, large birds use soaring and gliding whenever they can. They use flapping mainly to move from perch to perch and to assist in their takeoff. These birds beat their wings only from one to three times per second, while most songbirds beat about twice that fast. A hummingbird, just two inches (5 centimeters) long and weighing only one tenth of an ounce (3 grams), beats its wings 60 to 70 times per second. It can hover like a helicopter and is the only bird that can actually fly backward.
The Art of Turning and Landing
The control that birds have in the air is amazing. They can turn by beating one wing more rapidly than the other. This also causes the wing to be lifted up, enabling the bird to turn quite sharply. The tail feathers also play their part in this. Additionally, they help to provide balance and act as a brake when needed. The way birds dart in and out, dodging branches and avoiding near collisions with one another, shows that they are indeed masters of the air.
As far as landing is concerned, birds possess all the essentials to accomplish almost unbelievably perfect landings. A bird must consider its height, speed and direction and any wind currents so that it does not hit the ground hard or topple over on landing. Some heavier birds must run along a few feet to keep their balance.
Birds skillfully use their wings and also their tails to reduce speed and control the landing, enabling them to descend on a branch in such a way as barely to disturb it. This is quite an acrobatic feat when one considers the speed at which they approach the landing spot. Sometimes birds actually beat their wings opposite the direction of flight to slow themselves down quickly.
That birds are designed for flight is very evident when we look at their bone structure and covering. They have an upper arm fitting into a shoulder joint and a two-boned forearm. Because of the way the bones are connected, they move freely up and down and can also turn. The bird’s chest bone, instead of being flat like ours, is shaped like the keel of a boat. This provides a large area for the specialized and extremely powerful flight muscles to be attached on both sides of the chest bone.
The bones themselves are ideally designed. They are mainly thin-walled tubes, or, in the case of larger birds, are delicately strutted on the inside. As a result, the bones provide light yet extremely strong support. For instance, the whole skeleton of a frigate bird, with a seven-foot (two-meter) wingspan, may weigh as little as four ounces (113 grams). The larger bones also contain air sacs. These sacs are ancillary, or supplementary, to the lungs and, as needed, provide an extra oxygen supply for sustaining a bird’s high rate of activity.
The feathers, too, are a marvel of design. A bird may have from 2,000 to 6,000 of them. Each feather is equipped with hundreds of barbs branching out from a hollow shaft, and every barb has hundreds of barbules that branch into many even more minute hooklike barbicels. It is estimated that a single six-inch (15-centimeter) pigeon feather has about 990,000 barbules and millions of barbicels. All of these interlock to provide a most efficient airtight supporting surface that is lightweight, heat retaining and waterproof. The feathers also furnish a greatly increased wing area with very little additional weight.
There are three main types of wing feathers. The largest, the primaries, are around the wing tips and are very important for lateral steering as well as for flapping flight. The primaries in birds of prey change to a smaller width about halfway out. Apparently these birds can rise in the air at a much steeper angle because of this, enabling them to make better use of the natural air currents. The secondary feathers are attached to the lower arms, and the tertials to the upper arm. These types of feathers have their important roles to play in flight.
Miracle or Blind Chance?
Just a brief insight into the flight of birds makes one stop and think. Man has been able to imitate some aspects of bird flight after many decades of intensive design, experimentation and intelligent analysis. Still, he must rely on sophisticated instruments to do what birds do even better by instinct. Although man can produce gliders and now supersonic jet planes, he has been unable to imitate precisely the flapping of a bird’s wings, which can produce both propulsion and lift. How could bird flight, so dependent upon many complex factors, have originated?
Some claim that birds somehow evolved from reptiles, scales slowly turning into feathers. They point to the fossil of an ancient bird called the Archaeopteryx, which had teeth and a long bony tail, and claim that it is a “missing link.” However, a number of critical aspects are ignored. Reptiles are cold-blooded and often sluggish, whereas birds are warm-blooded and are among the most active creatures on earth. Flight depends upon many coordinated factors being present at one time.
It is noteworthy that Archaeopteryx already had fully developed wings perfectly feathered (not scales half developed into feathers), and had special feet equipped for perching. The relative proportions of the head and brain case are those of a bird and are quite different from those of reptiles. So, Archaeopteryx did not evolve from a reptile to a bird.
Surely, the ability to fly cannot be attributed to mere chance. Thoughtful study provides convincing evidence that bird flight is of divine origin. Everything about birds—their streamlined bodies, large, light wings, special bone structure, and all the necessary instinct to handle the complexity of flight—bespeaks an intelligent Designer far superior to man. Yes, our reverence should go to this One, Jehovah God, for the miracle of bird flight.—Ps. 148:1, 7, 10.
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the forces operating on a bird’s wing
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