Introduction

Ever since recorded time, man has watched birds fly through the sky and marveled at the ease with which they accomplish this feat. They take off, fly with twists and turns, soar and dive, and land again on a tiny branch, all with effortless precision. Fossil record shows birds have been flying for millions of years; although, it has only been within the last hundred years or so, that man has truly begun to comprehend the complexities of flying. In 1901 Wilbur Wright remarked, "that a bird's skill as a flier is not apparent...We only learn to appreciate it when we try to imitate it." No matter how hard he tried to fly, man had to unravel the mysteries of flight. He had to first understand the forces on earth that enable flight, and then to try and duplicate a bird's actions. It wasn't until technology enabled him to developed an internal combustion engine that he overcame the difficulty of minimum weight and maximum power, something nature conquered long ago. Also, the invention of the slow motion camera enabled man to capture the split second motion of a bird's wing in flight. He could now see what had been impossible to see before, and could master controlled flight. All of nature's secrets of flight, man has conquered with his brain and technology.

Nature first conquered the skies with insects, 400 million years ago. Their fast-flapping flight enabled them to successfully span the globe; however, because of their size, insect flight is haphazard and not generally considered controlled flight. A bird, on the other hand, is a perfectly controlled, very sophisticated, natural flying machine. Propellers, wings, flaps, and stabilizers are all secrets birds alone have shared for a very long time. What is it about a bird that has enabled it to perfect the art of flying like no other animal or machine has done this far?

There are many things about birds that make them good subjects for study. They are very abundant, so they are easily observed. They adapt easily to captivity and experimentation. Their great diversity with beautiful colors, characteristic songs, and unique habits makes each species easily identifiable. In fact, it is probably the great diversity among birds that enabled scientists to truly understand the complex combinations each species possesses and still achieves flight. Every thing about every bird is designed for flight no matter what combination of attributes they possess.

In order to understand natural flight in birds, one must first examine the structure of a bird. Exactly what makes a bird a bird? They all possess common characteristics. Next, one must explore the earliest birds and look at the special adaptions that enabled birds to achieve flight. Finally, one must look at the actual mechanics of flying to understand how birds take off, fly, and land. Occasionally, references will be made to the forces on earth that enable flight to occur.

What Makes a Bird a Bird?

Birds are unique in the animal kingdom for two reasons. First, all birds have feathers. And second, all birds live in a hurry. Everything about a bird is fast: they breathe faster than any other animal, their heart beats faster, and their body temperature is higher. Birds are vertebrates; they have a backbone. Like mammals, they are warm blooded; their body temperature remains constant regardless of the surrounding conditions. Like other vertebrates, except most mammals, they are oviparous. That is, they lay eggs. Unlike most vertebrates, they have no teeth. Instead, they have a bill or a beak with which they gather food and defend themselves. All birds have wings which are modified forelimbs, and their primary mode of locomotion is flight. However, all birds cannot fly. But it is the ability to fly that characterizes birds in most peoples' minds.

Birds are the most varied and the most successful of the vertebrates. There are about 8,600 species of birds living today. They are found everywhere in the world, from the tropics to the poles. However, each species has it own range: determined by oceans, climate, availability of food and nesting places. No one bird is found everywhere. But every bird has special adaptations to fit their unique life style and niche.

As inhabitants of the earth, birds have a special place in the ecological world. They play a vital role in the balance of nature: eating insects, agricultural pests, and other small animals; while they, in turn, are eaten by many larger animals. Fruit eating birds, because they do not digest seeds, are the primary seed dispersal mechanisms for many plants. Hummingbirds help pollinate nectar producing flowers. Seed eating birds do digest seeds, and by so doing, eliminate millions of weeds from the earth.

For man, birds have other values as well. They are sources of protein as both eggs and meat. Feathers are used for pillows, quilts and clothing. Down feathers are particularly good insulators. Pigeons and starlings are considered pests because their droppings contain bacteria harmful to man; however, there are other birds whose droppings are mined for fertilizer. Also, many people keep birds as pets.

Finally, man's fascination with the shear beauty of the bird has long been an inspiration for him. He has written poetry, stories, songs, and myths. Birds have also become symbols for human values: the owl signifies wisdom, the dove peace, and the eagle political and military power. The bright, colorful feathers of birds have been used for centuries for ornamentation and clothing.

Physical Characteristics of a Bird

Although man has observed birds for thousands of years, the scientific study of birds, called ornithology, did not begin until sometime in the 1700's. Using only direct observation, many physical characteristics of a bird are immediately apparent.

Feathers

The most noticeable feature of a bird is its feathers. Feathers are perfectly designed or their function. They are light but very strong, and they are flexible but very tough. Feathers do not grow all over the bird. Beaks and eyes have no feathers and most birds have featherless legs and feet. Scavenger birds that feed on dead animals, such as vultures, have very few or no feathers on their heads and necks because they would get dirty when they bird ate. Also, ostriches and the sacred ibis have featherless heads and necks because the live in hot climates and need the bare skin to get rid of excess body heat.

A bird's body appears to be covered with feathers, but not so. Feathers grow only in certain areas called feather tracks. The contour feathers overlap so they appear solid, but limiting the number of feathers helps to keep down the body weight. In between the feather tracks, the down feathers or the semiplumes grow. It is interesting to note, that in flightless birds like the ostrich and it's relatives and also in penguins, feathers grow all over the body.

Why are feathers light? There are several reasons.They are made of a tough, flexible material called keratin. Since it is not living, it doesn't need blood, nerve, or tissue supply. Feathers also look solid, but they are not. The spine down the middle, called the shaft, is hollow. Strengthening struts run across the hollow core like steps on a ladder. The vanes, which are the two halves of the feather that spread out from the shaft, are made of thousands of branches called barbs. These angle toward the tip of the feather. Each barb has tiny parallel branches called barbules. Because a feather has all these tiny parts with spaces in between, a feather has as much air as matter.

How do feathers grow? Each feather grows from a papilla or bump in the skin, which is surrounded by a pit of cells consisting of several layers. Each papilla is supplied with a vein and an artery. Growth begins with the tip of the feather. As an embryo the tip grows out toward the skin and eventually breaks through. When a bird hatches, the tip separates and appears as fuzz on the baby bird. This soft down is not a real feather, but works as a temporary covering. After hatching many changes take place in the papilla which then begin to produce the real feathers. The thin ,outermost layer forms a tough, protective sheath. Inside this sheath, the contour feather begins to grow from the pulp cells. First the central quill is formed. As the tip continues to grow, the downy fuzz is pushed ahead of it. Each feather is tightly rolled inside the sheath. It is called a pin feather at this stage because it appears long and pointed. The feather continues to grow and the pulp, vein, and artery follow along inside the quill to make the feather longer and broader. When the feather is full grown, the sheath splits open, falls off, and the feather uncurls. The vein and artery are sealed off in the papilla. The feather dies and the pulp shrivels, leaving a hollow shaft. The quill is stiff but springy and tightly anchored in its socket, surrounded by skin and muscles. Months later, when the feather is worn out, the process begins again.

A bird's survival depends upon the condition of its feathers. Consequently, they spend a lot of time caring for their feathers, a process called preening. Using their feet and beaks, they arrange the feathers to keep them smooth. Wind and brushing against things "unzips" the barbs, so they nibble each feather from the base to the tip to realign and hook the barbules. Birds also bathe frequently, because dirt adds weight. Most birds have an oil gland at the base of their tail and frequent oiling keeps the feathers water-resistant and shiny.

Eventually feathers wear out and need to be replaced. Molting, the process of loosing old feathers and growing new ones, occurs in most birds once or twice a year. Feathers are lost in a specific order and at varying rates, depending upon the species. Most take about two months to molt. Predatory birds molt slowly because they need most of their feathers to fly to hunt. Some of their flight feathers last two or three years. Other birds, like penguins, loose all of their feathers in a two week period of time, after the new ones have begun to grow. Water birds loose all of their flight feathers at once. They must hide until the new ones come in. During breeding season, most birds loose the feathers on their lower breasts and thus take advantage of the bare skin to warm their eggs.

Birds have between 1,000 and 25,000 feathers, depending upon the species. Obviously, larger birds have more feathers; the swan, with its long neck, having the most. Feathers can be divided into six categories. The contour feathers are the most abundant and cover the outer surface of the bird, giving the smooth, sleek profile so important to flight. All contour feathers have the same basic structure, with modifications depending upon placement and function. There is a central rachis, or shaft, which is hollow. Inside, dried remains of the pulp form strengthening struts which run crosswise like ladder rungs. The vanes are the two halves of the feather that spread out from the shaft. They are made up of hundreds of branches called barbs, angling toward the tip of the feather. Each barb has tiny, parallel branches of its own called barbules.

The vanes are smooth because of the structure of the barbs and barbules. The barbules on the edges of the barbs facing the tip of the feather, have microscopic hooks. The barbules on the side facing the base of the feather have rounded ridges. The hooks on the barbules of one barb catch on the ridges of the next barbule, holding the barbs together in a zipper like fashion. It is in this way that the solid looking, almost airtight surface of each feather is formed. As the feather bends and twists during flight, the barbule hooks slide back and forth retaining the smooth but flexible shape. However, a reverse twist from the tip to the base will separate the barbs and make the feather ragged with spaces in between. Preening "re-zips" the barbs and barbules. The barbs at the base of a contour feather have no hooks and so they appear fluffy.

The second type of feather is the semiplume. It is shaped like a contour feather, but its' shaft is not as stiff and its' barbs have no hooks so are fluffy.

The third type is the down feather. It is fluffy like the semiplume but has a very short shaft. Both the semiplume and the down feathers are important for keeping the bird warm. Air becomes trapped between the fluffy barbs and is held against the skin by the overlying contour feathers, thereby, preventing loss of body heat.

Some contour feathers combine the advantages of the down feathers on a single feather. They have an afterfeather, which is a second shaft coming from the main quill like a branch. It is fluffy. One feather performs two functions: smooth covering as well as insulation while decreasing overall weight.

The fourth feather type is called the filoplume. These are tiny and delicate with only a few barbs on the tip. They are sparsely scattered amoung the other feathers. Because there are nerve endings at their base, scientists believe their purpose is to sense the positions of the contour feathers, so the muscles in the skin can keep them in place.

Bristles are stiff, hairlike feathers found only in some birds. Their function is specific in each species. Bristles are found around the mouths of birds who scoop insects out of the air. There are several theories as to their function, but it is now thought the bristles help keep the insects out of the bird's eyes. Bristles are found covering the nostrils of woodpeckers, presumably to help keep the wood dust out of their nose. In ostriches bristles form eyelashes.

The final feather type is found only in a few birds. It is the powder-down feather. This feather grows continually. The tip breaks off forming a water resistant powder. The metallic sheen of the heron is caused partly by this powder down.

A bird's wing is the basic structure for flight. It is covered with contour feathers that are specialized for flight. It is the shape of the wing that enables a bird to fly, and the shape is determined by the feathers.

The actual wing is a modified forelimb, with a skeletal structure like an arm. It is broad and mostly feathers, with very little skin and bone. The upper bone, closest to the body, is where the flight muscles attach. The actual wing is V-shaped with two bones in the next section. The outer portion of the V is made up of long wrist and fused finger bones. Part of the thumb is present as a projection called the aulula, which protrudes from the front of the wing. This is very important during flight and will be examined in more detail in the mechanics of flight.

The feathers on the wing are the flight feathers, specialized contour feathers. They have an especially rigid shaft. The vanes provide a light weight, broad surface that pushes on the air to make flight possible. There are three kinds of flight feathers on each wing. The primaries are attached to the outer part of the wing, to the hand and finger bones. There are between nine and twelve on each wing. The inner part of the wing has the secondaries, Again the number varies by species: hummingbirds have 6 of 7 and large birds have as many as 32. The tertiaries are attached to the upper wing.

It is critical that air flows evenly around and over the wings during flight so that friction and drag are kept at a minimum. The surface of the wing is kept smooth by the overlapping placement of the flight feathers. Each feather is also shaped so the the side facing the wind is narrower and stiffer than the trailing edge, which makes it stronger. In addition, along the leading edge (front) of the wing, are smaller contour feathers, called coverts, which cover the base the flight feathers. These feathers give the wing the airfoil shape that make flight possible.

Although the curved airfoil shape of the wing is necessary to lift the bird into the air, feathers contribute much more during flight. On the upstroke of the wing, the feathers tilt so that air can pass between them, lessening drag. On the downstroke, the larger vane of the trailing edge bends upward, pushing down the leading edge. Each feather performs like a propeller with every wing stroke. In addition, with every stroke, the wing is positioned either slightly forward or backward, not just up and down. The hummingbird can move his wings in a figure 8 pattern which enables him to hover. Large, slow flying birds have additional features on their flight feathers.

In order to prevent stalling, which occurs when the airspeed over the wing slows, each flight feather has a notch on the leading edge, about 2/3's of the way to the tip. The narrowing of the vane leaves spaces between the feathers when the wing is spread which helps even out the airflow. Also, these birds can spread the feathers on the alula to create and additional slot. Fast flying birds do not have these notches on their feathers.

Feathers give the wing its shape and there is a direct correlation between form and function. Birds who fly fast in open air have long, narrow wings. They have difficulty taking off, but can stay in the air indefinitely, once airborne. Woodland birds must fly slowly to maneuver between branches and trees, as well as take off frequently, have short, broad wings and wide feathers. They cannot fly as fast or as long as birds with longer, streamlined wings. Finally, birds that soar, have broad secondary flight feathers which greatly increase the surface area of the wing so they can ride easily on the warm air currents.

The last place feathers play a vital role during flight is on the bird's tail. The tail acts as the rudder, balancing and steering the bird. The tail feathers have vanes of equal size and can be tipped in different directions for stability. While soaring, the tail feathers are spread to increase the surface area and get more lift. The entire tail can be twisted to change direction. And last, to aide the bird when stopping. the tail is turned downward and acts like a brake.

It is truly amazing the critical role feathers play in the flight of a bird. No other animal or machine has duplicated the intricacies of a single feather. But feathers serve other functions too. They protect the bird's skin from cuts, bruises, sun and rain. Predators often end up with feathers instead of bird for their meal. Woodpeckers have extra stiff, pointed tail feathers which they use to stick against the tree for additional support while they drill for insects. The sand grouse in Africa soaks up water with its breast feathers to feed to is babies. Feathers insulate. A penguin's body temperature is 110 degrees, while the surrounding air can be -75 degrees so their feathers cover their entire body and are almost hairlike. Feathers can be fluffed up in the winter or squeezed down in the summer. Some birds grow more feathers in winter. Ostriches can make their contour feathers stand up to release excess body heat.

The list of adaptations of feathers to suit a particular bird's life style is endless. Feathers are used to line nests which helps to hold in heat. Song birds collect old feathers, sparrows pull feathers from other birds, and ducks and geese pluck their own down feathers and then use the bare skin on their breast to warm the eggs. Hatchlings that are tree nested are born naked and, therefore, must be cared for by the parent. Those who nest on the ground are born with fluffy down, are not dependent upon the parent and can feed themselves. Water birds have special oil glands to make their feathers more water resistant. The exception are birds that dive; they have no oil glands because they cannot be buoyant. They must dry their feathers in the sun.

Owls are nocturnal hunters. The undersurface of their wing feathers are fluffy and velvet-like, the leading edges have unlinked barbs, and fringed trailing edges, all to muffle the sound of the air as it passes over the wing. They also have very densely packed face feathers which help the owl gather and focus sound so they can hunt in total darkness. The final significance of feathers is appreciated not only by the birds themselves, but also by man. The variety of colors found in a bird's plumage is not only magnificent but functional. Because birds fly, they don't need as much camouflage and can have much more colorful skin coverings.

Color is important in mating. Birds can see color, where most mammals do not. Colored plumage is used to attract a females attention or is used to be visible to other males as a warning. The amount of color found in different birds is dependent upon life styles. If both male and female sit on the nest, both will have dull brown coloration. Feathers used to camouflage can have disruptive coloration which are spots or patches of color, they can totally change color (like the Ptarmigan in winter) or that can utilize counter shading where the top of the bird has darker feathers and the underside tends to be a lighter color. In the tropics where there is lush vegetation and dense foliage, birds have the most brilliant colors because there are plenty of places to hide.

How are the colors produced in feathers? Light comes from the sun in waves. Each color has a specific wave length. When something appears colored, the object absorbs all the wave lengths of light except the one it reflects. The reflected light is the color one sees. Substances that produce colors are called pigments. Black, brown, some red, and dull yellow in feathers are produced by pigments called melanin. Melanin are microscopic particles found in the shaft and barbs of feathers. Black melanin is rod shaped but the others are oval shaped. The more melanin particles present, the darker the color. Feathers with lots of melanin are stronger. Consequently, it is not uncommon for white birds to have wing tips that are black, because the wingtips are constantly battered by the wind and need to be stronger.

The brilliant colors- red, gold, yellow, and orange - are formed by pigments called carotenoids. These are not individual particles, but, rather, pigments dissolved in fat globules that are left behind when the feather stops growing and dies. Birds that are colored by carotenoids use the pigments in their food and change them into the form used in feather formation. Flamingos are perhaps the most widely known birds whose feathers are colored with carotenoids.Their natural diet contains the pigment that makes their feathers pink, but when in captivity, zoo food lacks the proper pigment and their feathers are white after they molt.

The color blue is not produced by pigment. Rather, it is produced in the same manner that makes the sky appear blue. There are tiny, air filled pockets in the feathers that scatter the blue wave lengths. In the barbs of a blue jay's feather, these hollow cells form a layer over the melanin. The melanin absorbs all the other colors, making the blue appear more intense. Green is produced in a similar manner, only a transparent layer of yellow cells lie over the particles that scatter the blue wave lengths.

White is a mixture of all colors and white feathers have no pigments at all. Some, however, do have air bubbles that reflect back the light making the white appear even brighter.

The iridescent colors found in hummingbirds and peacocks, are produced in yet a different way. In peacocks, the barbules of the tail feathers have three very thin layers of keratin which reflect the colors much like a soap bubble does. In hummingbirds, the barbules are twisted so the flat sides face out. These are coated with at least three very thin films of oval, plate-shaped structures, which hold tiny air bubbles. These reflect the light in different wave lengths producing the iridescent colors found in the feathers.

Feathers truly make birds unique in the animal kingdom. The secrets found in a bird's feather is awesome.

Beaks And Feet

The bill, or beak, is another unique external feature of birds. Instead of a heavy jaw bone and teeth, birds have a lightweight beak made of a horny material. Because a bird's forelimbs are used for flying, most birds must be able to obtain their food with only the use of their beak. The shape of a bird's beak varies with the type of food it eats as well as the bird's method of feeding. For example, seed eating birds have sharp, cone-shaped bills which they use like a nutcracker. Fruit eaters have a similar bill but they use the sharp point to pierce the skins of the fruit. Woodpeckers have chisel shaped bill for drilling. Ducks, who feed on floating plant matter, have wide, flat bills that have tiny filters along the sides. They can take in large amounts of water and the filters trap the food while the water drains out. Other bill shapes include long, pointed ones for spearing fish, scoops and pouches like a pelicans, and trap bills that open very wide to catch flying insects.

All birds have two legs and two feet. On the ground, most birds get around by walking, hopping, or climbing. The shape of the feet and legs is also dependent upon use and function. Short legged birds spend most of their time aloft. Fast runners and wading birds have long legs. Most birds have four toes on each foot; three facing forward and one curling behind. This configuration allows a bird to grip a branch to perch. Birds that climb. woodpeckers and parrots, have two toes forward and two backward. This gives the extra gripping power. Most flightless birds have three toes, but the ostrich has only two. Swimming birds have webs of skin between the three front toes to make a paddle. All birds have claws on their toes, but it is the birds of prey in which they are most prominent.

Internal Structures of a Bird

Flight is much more than just feathers and wings. How is a bird specifically designed for flight? Everything about a bird is perfectly designed for flying. Modern birds are structurally and functionally efficient because they must be able to take flight, stay aloft, and reach their destination under many diverse conditions.Their skeletal, muscular, nervous, circulatory, respiratory, digestive, and reproductive systems have all been designed for flight. All systems must exhibit maximum power and efficiency with a minimum of weight.The key factor in adaptations for flight is lightness.

First of all, there is a weight limit for a flapping bird. The heavier the animal, the bigger its wings need to be. The bigger the wings, the more muscle is needed to move them. This again increases weight. Also, the bigger the wing, the more air resistance there is on that wing. More stress is applied to the wing; therefore, it must be stronger.

The biggest flying bird today is the Great Bustard of Europe and Australia. They weigh as much as 32 pounds and are four feet long. The largest bird ever to fly is extinct, but its bones were found in Nevada. It's scientific name is "Teretornis incredibilis". It's wing span was 17 feet. There have been other giant birds, but none we know of were able to fly. Today, the largest bird is the ostrich, but its 8 foot height is not the record. The extinct Moa of New Zealand stood 12 feet and weighed 700 lbs. The elephant bird of Madagascar weighed 900 lbs but wasn't as tall. Scientists guess these birds grew so big and no longer needed to fly because there was plenty of food on the ground and there were not many enemies to chase them into the air or prevent them from nesting on the ground. Walking and running were more important than flying, so their legs became heavier and stronger.

To fly, a animal must be light, and the easiest way for this to happen is to be small. Another way to make an object lighter is to use light weight materials to build it. However, these light weight materials must also be strong enough for flight. Birds have feathers that are very light weight. Feathers create a smooth, hard, streamlined surface. A bird's wing is almost all feathers, with only a few bones along the leading edge.

Bones are also designed for lightness. Birds have fewer bones than most animals. The bones they have are hard but thin and many are hollow and filled with air sacs which are much lighter than solid tissue (like corrugated card board). The wing bones are long and thin but very strong. Some bones have diagonally placed struts in them for added strength. Bent or braced materials are often very strong for their weight. A bird's skull is paper thin. Also beaks are very light weight; even a Toucans' beak is made of extremely light and porous layers of bony material.The biggest bones in flying birds are the breast bone and the shoulder bones. The muscles that move the wings attach to these bones.

Comparing the weight of a birds skeleton with the weight of the rest of his body, is a good way to tell how well a particular bird flies. Generally the best gliding and soaring birds, have the lightest skeleton. Some have bones weighing less than their feathers. The powerful flying birds have medium weight skeletons. Non-flying birds have the heaviest skeleton.

Another weight reduction scheme is reducing the number of muscles a bird needs. For example, in birds many bones are fused together so that muscles are not needed to hold them in position. There are fewer joints making the skeletons very ridged.

The biggest muscles a bird needs are its flight muscles. These are the most highly developed because they must raise the entire body weight into the air. A human can only do push-ups only when his arms are directly below him, and even that's difficult. Spreading our arms sideways makes push-ups impossible, because our chest muscles are not developed enough. A man's chest muscles equal 1% of his body weight, whereas in birds it is 15% of the body weight. The major and minor pectorals of a bird move its wings. Very little muscle is found in the wing itself, for that would add weight and make the wing even more difficult to move. Instead birds muscles are in the chest and are attached to the wings by large tendons. The major pectoral attaches to the under side of the wing bone. The minor pectoral attaches to the top of the wing bone by a long tendon that passes over the shoulder like a rope over a pulley.

The breast bone in birds has become enlarged, shaped like a keel, and very strong to support these powerful flight muscles. Generally, the size of the keel is proportionate to the flying power of the bird. So the more wing muscle, the larger the keel. Non-flying birds do not have well developed pectoral muscles and, therefore, have flat breast bones. Not all muscles in birds are used for flight. Most birds also walk or hop or swim, so they also have well developed leg muscles.

In conjunction with the skeletal and muscular adaptations for flight are structural adaptations. The entire shape of a birds body is streamlined. Feathers complete the smooth surfaces. There are even bays for the feet to be withdrawn during flight to decrease drag. Balance is achieved by the central positioning of all the heavy, locomotor muscles at the body's center of gravity. The wings are controlled only by tendons. Also, the gizzard and other internal, abdominal organs are located low, within the center of gravity.

Other internal systems are uniquely designed for birds. A birds' reproductive system is even suited for flight. Mammals carry their babies internally while they are developing. Reptiles lay egg. Birds lay eggs but they protect them in the nest while the embryo develops. Egg formation within the female is very quick. In a chicken formation takes 26 hours. As soon as the egg is finished, it leaves the females body. Birds are not like most other egg layers, reptiles and fish. They collect large numbers of eggs in their abdomen before depositing them. If more that one egg is to be laid, they are produced one after another.

The brains of birds are smaller than those of most mammals. They live in a world of sight and sound so those areas of the brain are most highly developed. The visual and locomotor area must be capable of transmitting nerve impulses very quickly because birds fly at high speeds. They must judge distances accurately to make landings, catch insects and other prey, distinguish colors (day hunters), see in dim light (night hunters), and see from great distance(high-flying birds). Consequently, a bird's brain is significantly more developed in the area of vision than in any other area.The occipital lobe and the eyes fill most of a birds skull. Some birds even have eyes that are so large, they weigh more than their brain.

Most birds have eyes placed on the sides of their heads. This greatly increases their field of vision. However, because the eyes are fixed, they must turn their heads to see more. Birds have very flexible necks.

The structure of their eyes is also unique. Birds have three eyelids. There is an upper lid like humans have. There is also a lower lid which is usually closed when sleeping. The third lid sweeps horizontally across the eye cleaning it like a windshield wiper. It may also protect the eye against wind while flying. In diving birds it acts like an extra lens. Hawks and owls may be able to bend this third lid like a lens, thus, giving them the same effect as a zoom lens on a camera.

The actual image a bird sees depends on the placement of the eyes. In binocular vision, the two eyes face forward and focus on one object, giving a 3-D image. Owls and humans have binocular vision. Monocular vision occurs when the eyes are placed on the sides of the head. Each eye sees a different image. The advantage here is the field of vision greatly increases, so that the bird can see danger from both sides. The biggest disadvantage is that the images produced are flat and judging distance with one eye is very difficult. Many birds have both types of vision. The fields of vision cross over and they can see 3-D directly ahead as well as sideways.

The eyeballs of birds are flatter than those of humans. That means they can see more things at one time in focus. The muscles in bird eyes are quicker and stronger, and the lenses are softer; therefore, they can change focus from one object to another much more quickly. Birds also have larger eyes so they can see more at one time. Finally, the retina of a bird's eye is 2 times as thick as a mans' . It contains many more rods and cones enabling a bird to see much more detail than humans. A sparrow hawk sees 8 times more clearly than man. Not surprisingly, the area of a bird's brain controlling vision is highly developed and very large.

Hearing is another sense that is well developed in birds. The ears not only hear well but are also responsible for maintaining balance. This is of critical importance during flight.

The cerebellum, the muscular control area of the brain, is also extraordinarily well developed. Flying is such a complicated series of muscular movements, the brain developed accordingly.

Internal Power Systems

Becoming airborne requires a tremendous amount of energy and taking off requires far more than moving forward. In addition, birds who are warm blooded animals, must have extra power to maintain a constant body temperature. Small birds have more difficulty keeping warm than larger ones. So a large bird burns extra energy taking off and a small bird uses extra energy keeping warm: they both need a well designed mechanism for supplying energy to their bodies, while maintaining lightness.

Birds have a natural mechanism to solve this power problem. All internal organs of a bird run at high speed. This produces high energy, but also shortens the life span of the bird. Song birds live only about two years. The kinds of foods consumed by birds must be light weight and "high-octane". Foods high in calories produce the most amount of reusable energy: seeds, nuts, fruits, fish, and rodents. Most birds do not eat leaves or grasses. These foods are used up quickly, and some birds must eat their weight in food everyday to maintain themselves. The speed at which a hummingbird burns its food is 50 times greater than man. Most birds feed during the day and are able to replace the foods they use up. However, at night they must rest and their body functions slow down in order to conserve energy. A hummingbird would starve to death in the night if his heart, respiration, and body temperature didn't slow down. This torpid state works very much like hibernation.

Birds are able to "eat and run" because the wall of the esophagus is very thin and stretches easily so the food does not have to be chewed or torn up. The crop, at the lower end of the esophagus, has developed into a storage area in some birds. Some birds are not bothered by a meal too large for it's stomach; the head of a fish can be in the stomach and the tail in it's throat. The food can be held in the crop for hours. In some cases the crop acts like a first stomach where the food is softened and predigested. It is then passed on.

When the food enters the stomach, digestion begins. Very strong gastric juices begin the breakdown process. The lower part of the stomach, the gizzard, has powerful muscles for mixing and crashing. Since birds have no teeth, (another feature for lightness) birds that eat hard grains and seeds must swallow small stones or shells to help the gizzard grind the food. When the stones are worn down, they are past on with the waste products. The chemical breakdown continues while the food moves into the intestine where it is absorbed by the blood and transported throughout the body. This digestion process is very rapid (20 minutes). There are no waste storage areas, so particles that can not be used are immediately passed out of the body. Some of the birds of prey eat their food whole. The indigestible bones, fur, and scales never go past the stomach. Instead they are coughed up as a small pellet.

To release energy from fuels, food, gasoline, or coal, they must be burned. This is called combustion and requires oxygen. This chemical change produces heat. The more oxygen available, the faster the fuel burns. Oxygen is taken into the body through the lungs where it passes into the bloodstream and is carried to the blood cells all over the body. In the cells it combines with fuel and burns to produce energy. The wastes are then carried back from the cells via the bloodstream. The CO2 is released through the lungs and the liquid and solid wastes go through the kidneys, intestines, and out. The process is similar in both birds and mammals.

Birds need a much more efficient system than humans because their digestion process must happen very quickly to supply the energy they need. They have a system of air sacs connected to the lungs that extend throughout the body and into the hollow portions of the larger bones. There air sacs are light as well greatly increasing the amount of air a bird can hold internally.

Not only do birds lungs work faster than ours, they work very differently. Birds never run out of breath; instead, they fly into breath. In man the diaphragm contracts, expands the lungs, and draws in fresh air. Then, the diaphragm relaxes, the lungs return to normal size, and we exhale. In birds the respiratory system is the opposite. The stale air is released when the muscles contract and fresh air is drawn in automatically when they relax. They have no diaphragm, another weight saving feature. Instead they use the chest muscles that power the wings. In flight the pectoral muscles contract to move the wings, pressing against the ribs. The bird is forced to breath faster as it flies faster.

The biggest difference is how the air moves through the two systems. Man's lungs are like a upside down tree. Air moves through the branches down to the tiny air sacs. There oxygen and carbon dioxide are exchanged through the blood. The oxygenated blood is circulated through the body and the carbon dioxide is exhaled on the next breath. Man inhales and exhales at opposite times. the exhale is also not complete; there is a small amount of carbon dioxide left in the air sacs.

In birds' lungs, the air moves continuously in one direction throughout the system of tubes and air sacs. These tubes are open at both ends so stale air can be flushed out more efficiently. There is always fresh air in the lungs, both upon inhale and exhale. The gasses exchange in the smallest of the tubes as the blood passes by. The birds respiratory system is like a jet engine.

In order to maintain the high energy levels necessary in a bird, the food and oxygen in the blood are very concentrated. Compared to man, they have 2 times the amount of sugar for burning. Although the number of red blood cells that carry oxygen is the same, those in the bird are much more efficient. The hemoglobin accepts and releases oxygen much more easily, so more oxygen can be delivered to the cells.

The heart is the key for pumping all the blood through the system. In general, a birds' heart is larger proportionately that other animals' hearts, because the pumping capacity must be greater to meet the extra energy needs for take off and landing. At these times, it must be able to speed up tremendously. The size of the heart depends upon the size of the bird and where it lives. The smaller the bird, the larger its heart. Because the smaller birds loose body heat much more rapidly, the blood must be pumped more quickly to counterbalance its energy expenditure. Birds that live high in the mountains where the air is thinner, or those that live close to the poles in colder temperatures, also have proportionately larger hearts. A bird's heart beats much faster under normal circumstances than other animals. This rapid pumping enables blood to travel through a bird's system in seconds.

A bird's circulatory system operates under great pressure too. This rapid pumping system gives off large amounts of heat. A bird's body temperature averages 7 or 8 degrees higher than mans'. This is approaching the upper limit for living things, so it is very important that birds have some way of getting rid of excess body heat.. In man extra heat is eliminated through perspiration on the skin. Engines use water in the cooling system. A bird cold down through evaporation from inside their bodies, A fast air flow is essential. One half of a birds air intake is used for heat reduction. A lot of heat is evaporated from the upper part of the lungs in a similar manner to dogs when they pant. But during flight, the lung surfaces are not large enough to expel all the extra heat produced. So the large air sacs must be used too.

The Mechanics of Flight in Birds

The distinctive shape of a wing is known as an airfoil. As the airfoil moves through the air, air goes above and below. Lift is created by the movement of the air around the wings (the lift created by the body or tail is small). The shape of the wing is important in generating lift. A bird's wing is "cambered" with its upper surface curved more than its lower surface. Due to its shape the pressure on these surfaces are different. This pressure differential generates the lifting force experienced by the wing. The greater the weight of the object the more lift is needed.

The amount of lift a wing can produce is governed by several factors. One way to increase the lift is to change the angle of the wing as it faces the air. Tilt the leading edge up and the distance the upper air stream must flow is even greater. This is called increasing the angle of attack. The bird changes the angle of the whole wing, where a plane lowers the flaps on the trailing edge, but the result is the same. If the angle of attack is to great all lift disappears and the bird or plane begins to fall to the ground. This point is called the stalling point.

There is still another way to get maximum lift without reducing the angle of attack. If the air across the wing can be made to move even faster, thus changing the pressure which pulls the air stream back down on the wing, then lift will be produced again. This can be accomplished by forcing the air through an even smaller space, or slot, just ahead of the upper surface of the wing. These slots can be formed in a number of ways:

1. In birds there are a few feathers attached to a movable finger bone like our thumb. It is called the alula and it is on the front of each wing. The bird can easily adjust this slot to increase lift.
2. Birds can also manipulate the feathers at their wing tips to produce slots if necessary.
3. Birds can also raise the feathers on the leading edge of the wing to form slots.
4. On an airplane there are slots that can be opened on the top of the wing. All of these ways are used to increase the speed of air moving over the upper surface of the wing.

Speed is the most important element in producing lift. However, speed can be increased by increasing the forward speed of the wing itself as it travels through the air. This causes a even more dramatic change in lift. If you double the speed, you get 4 times the lift. Triple the speed and you get 9 times the lift.

The weight of the bird (or plane) determines how much lift is necessary. Contributing to this is whether the bird is going up, down, or in level flight. Going up, lift must be greater than weight, in level flight, lift must equal weight, and going down, lift is less than weight. Therefore, birds with large wings get lots of lift at slow speeds, but birds with small wings must move faster before they become airborne. Minimum speed for lift is dependent upon the design of the bird.

Most birds must be moving forward through the air in order to get lift. So, how do birds create forward motion? In propeller planes, air is pushed backward as the propeller spins. In jets, air is forced through the engines and out the back. In birds, it is the flapping of their wings that produces the forward motion called thrust. But how does the up and down motion push the bird forward? One would think the downward flap might push the bird up, but the upward flap would push it back down again. To understand how a bird produces forward motion, the structure of the wing must be examined.

Comparatively, a bird's wing is very similar in structure to a man's arm. Man has 29 bones, most birds have 11. Man's hand is very complex, containing all but three of the bones. A bird's hand bones are much longer, fused together, and much simpler. With fewer bones, there are fewer joints, so fewer movements are possible. It is this rigidity that makes the wing so strong. It is the "hand" section of the wing that produced the power to propel the bird through the air. The fusing of the bones keeps the wing tip in proper alignment with the rest of the wing.

The birds "elbow" is also designed so that it cannot bend in the direction that takes the most amount of stress during flight. That eliminates the need for extra muscles to keep it in place, thus increasing the weight of the bird.

The "shoulder" joint is also designed so that the inner wing is automatically held at the proper angle of attack for maximum lift. This section between the shoulder and the wrist moves very little during flight. It also has the airfoil shape that the airplane wing was designed after. It is this curved surface that produces the lift as the bird moves forward.

The structure of the bird's wing is such that it can be folded close to the bird's body when it is not in flight. But when the wing is extended, it act as both wing and propeller. The feathers attached to the "hand" bones are the ones that produce the forward thrust for the bird.

The forward motion of flight is the result of pushing backward. When a person walks forward, he pushes off the ground. Rowing a boat forward requires pushing against the water. On a bird, the primary flight feathers are what pushes backward against the air. They can be likened to the blades of a fan, only they only go 1/2 revolution at a time. On the downstroke they push backward against the air. Then the feathers twist in such a way they push air backward on the upstroke too .The feathers twist back and forth as the wing goes up and down. What makes them twist is the off center position of the shaft. Flight feathers in the tail, with a central shaft position, do not twist.

Since the twisting of the feathers works in your hand, it is obviously not controlled by any muscles in the bird. The twisting is automatic and is caused by the unequal size of the vanes. The larger vane bumps into more air than the smaller one. On the upstroke, more air hits the top of the larger vane, so this part is pushed down. On the downstroke, again the air pressure is greater on the larger vane but this time it pushes it up, twisting the feather in the opposite direction. Thus, as the wing flaps up and down, the feathers twist back and forth. Only the primary and secondary feathers do this.

The secondary feathers make up the inside part of the wing and are very close to each other. When they open and close, they resemble a window blind. On the upstroke they open so the air passes through easily. This happens very quickly because there is very little air resistance. On the downstroke the feathers are closed, forming a almost airtight surface for the powerful, downward and backward push.

The primary feathers at the tip of the wing are the most important. First, they travel the furthest distance. Swing your arm. The hand travels further than the elbow. They are spaced so they don't interfere with each other when twisting. Third, they are stiffer than the other feathers. Their barbs are thicker near the base which gives them an airfoil shape. They also twist back and forth, but it is these feathers that give the bird its main forward thrust.

The downstroke consists of a specific series of events. The primary feathers, with the wider vanes on the top are twisted up, so that each of these propeller feathers are at the right angle of attack. As the breast muscles pull the wing down, each feather bites through the air and pushes backward against it. This produces thrust, just like a planes's propeller. As the wing moves down, because of its airfoil shape, a partial vacuum forms on the upper surface. Since the upper surface of each feather is facing forward, it is sucked forward by the vacuum ahead of it. So, on the downstroke, the primary (propeller) feathers are not only pushing against the air behind them but are being pulled from the front as well, dragging the wing forward.

The secondary feathers are also rotated into a closed position, so their entire undersurface is pushing down and backward against the air, helping to move the bird upward and forward. There is tremendous air resistance to this large, flat surface which slows the wings movement. Tremendous power is needed to overcome this resistance. The power comes from the large breast muscle. In most small birds the powerful downstroke is all that is needed to produce forward thrust.

At the bottom of the downstroke, the wings may be far ahead of the bird's body. The body catches up in a split second. This pulling ahead of the wings is most noticeable at take off, because the bird's body is also resisting forward movement. During flight, the body does not lag so far behind. On the upstroke, the large muscle relaxes and the small breast muscle begins to pull the wing upward. The second the wing begins to move up, the air twists the flight feathers and the wings open up, so there is very little air resistance. The upstroke is fast and easy. Some birds also draw their wings up very close to their bodies to have even less resistance.

For most birds, the upstroke is only a means for returning the wings to the starting position for the next downstroke. But, in some larger birds, the primaries are angled in such a way that they push some of the air backward. This gives a small amount of forward thrust. As the upstroke continues, it looks as if the wings are moving backward, but it is really the body moving forward. At the top of the upstroke, the wings are once again positioned for the next downstroke, except that the flight feathers are still open. Immediately, when the large breast muscle pulls, the feathers close and the cycle repeats itself over and over.

This single wing beat takes only a instant to occur. No wonder it took man so long to discover the secret of flight. It was actually the invention of the slow motion camera that finally gave man his first look at a bird in flight.

Gliding Flight

Some birds, like the Layson and black footed albatrosses, can glide and soar for hours. Eagles and vultures glide for miles, apparently not affected by the pull of gravity. Some even go up. How? If the power doesn't come from the wings, where does it come from? While soaring and gliding, birds take advantage of upward movements of air called thermals. These bird's wings are resting on piles of air that are moving upward. We cannot usually feel these updrafts on the ground. We can ,on very warm sunny days, observe that distant objects seem to quiver. This results from looking through waving currents of warm air rising from the ground. You cannot see the air itself; what you do see is the way it distorts the objects behind it.

Many birds find these thermals and use them for the extras upward push they give. Added to the aerodynamic force of the wings, there is enough force to overcome the pull of gravity, and no other flapping power is necessary. These birds then, are gliding downward on a constantly rising current of air. Warm air does not rise evenly from the ground. It is shaped more like a doughnuts piled upon one another. The air moves up the center and then falls down on the outside of the ring as it cools.Eventually the thermal thins out so it can no longer push the bird upward. When this happens, the birds glide down until they can catch another rising current of air. Riding thermals are great energy saving devices for birds when they can find them. But they don't always exist. The sun must have warmed the ground sufficiently. Many large birds wait on the ground until 9 or 10 o'clock until the thermals form.

There are other kinds of updrafts in addition to thermals. Obstruction currents form when moving air run into an obstacle, like a cliff, hill, mountain, or even a building. The air is forced to rise up and over the obstruction.

Sea birds use another kind if air movement. Ocean breezes, like trade winds, offer a steady source of power upon which they can rise and glide. This is called dynamic soaring.

All birds must use powered flight. Flapping wings provide the power to begin and end all flight.

Take Off and Landing

The most dangerous time in any flight (bird or plane) is during takeoff and landing, because speed is related to the lift necessary to leave the ground. Lift is weakest at take off because full speed had not been reached and at landing because speed is being reduced. There are other factors that birds and planes use to increase lift at these critical times.

First, the size of the wing determines how much lift is created. In planes the ailerons and flaps slide out of the back to increase the surface area of the wing. This gives extra lift at slow speeds. The angle at which the wing approaches the air also affects lift. Planes have flaps that they lower to increase this angle and ,thereby, increase the lift . Planes also take advantage of any winds that might be blowing by taking off and landing into the wind. This increases the air speed over the wing and produces more lift. All planes also have slots to increase the air speed over the wing. Even with all these tricks to help, takeoff is still difficult.

Birds use all these same techniques. They change their wing size, lower flaps, open slots, use existing wind, and increase the speed of their propellers by flapping their wings faster. However, every movement a bird makes, requires tremendous expenditures of energy, far more than just flying.

Larger birds have much more difficulty getting airborne than smaller ones. For example, the South American condor, with wing spans of 9 feet, cannot flap its wings while on the ground. They must use some sort of outside help to takeoff. They need a very strong head wind or more frequently , they land only on cliffs so they can leap off to begin their next flight.

There are a number of other aids birds use during takeoff:

1. Some birds can rotate their wings at the shoulders. Instead of the up and down movement, it is almost a back and forth movement. This requires a lot of extra energy, but it produces so much lift that the bird takes off more like a helicopter that a plane.
2. Birds can change the position of the alula to create slotting and increase the air speed over the wing. Some birds must use this technique.
3. Some birds have long legs which they use to push themselves into the air, giving them the space necessary to complete the first downstroke. The Heron is an example. Some ducks push against the water.
4. Most water birds - geese, swans, loons, grebes - need very long runs before they can obtain enough speed for flight. Their large, webbed feet make running on water possible.

The most comical takeoff and landing is made by the albatrosses in the South Pacific also called "Gooney Birds". They run, flap, hop, and finally with the aide of a head wind manage to get airborne. Landing is another problem. Thermals and breezes are great high up, but the closer the bird gets to the water, the slower the wind moves and the weaker the updrafts. The gooney bird looses altitude very rapidly until it comes in for a crash landing on its nose!

Landing is even more difficult than takeoff. Flight must be ended gradually. The heavier the bird the greater its speed and the more difficult the landing. There is a specific sequence for landing. First, the bird slows its wing beats, reducing the forward speed and lift. Next, gravity begins to pull the bird down. If the bird is gliding with outstretched wings, the primary feathers are no longer used to push forward, but are now twisted to increase the surface area of the wing. This creates more lift to slow the birds descent. At the same time, the whole wing is rotated slightly at the shoulders to increase the angle of attack. This creates additional lift as the speed continues to decrease preventing the bird from dropping too quickly. Next, the tail may be spread open and lowered to act like a brake.

The feet are forward, ready to grab the branch. To slow it even more, the wings may be cupped like parachutes. The slots are opened to prevent stalling.

If the bird is still approaching to target too quickly, it can put its propellers in reverse. By twisting its wings slightly at the shoulders, air pressure automatically bends the outer primaries up at the tips. Now when the wings swing forward the feathers push up on the air, in the opposite direction that they normally do (propel in reverse). This acts as another brake in the forward motion.

The moment of impact is still dangerous. The branch may move in the breeze, a gust of wind may roll the bird, so another wing flap must be made to make corrections. If they are already among the branches they must be careful not to injure their feathers. The feet finally grab on and the strong leg muscles absorb the final shock. Flight has ended. All of this takes place so quickly that man sees, but doesn't really see.

Variety in Flight

Birds live under all conditions and in all places on earth. This diversity of habitats has created birds of many different kinds, each with special needs and adaptations to meet those needs. Size is a critical factor. Some birds are too heavy to fly, like the ostrich and the EMU. The heavier the bird, the more lift it needs to fly. To meet this need, wings come in different sizes.

There is also great variety in speeds at which birds fly. The top speed of a bird depends upon its design, and this design is determined by where the bird lives and how it gets its food. Generally, the larger the bird the faster it flies. One interesting fact is that the rate of the wing flap does not determine speed. The vulture, whose wings flap once per second, has a very powerful thrust. A small bird flaps it's wings 4 times per second and flies at 25 mph. The hummingbird flaps its wings 10 times per second and flies at 60 mph. Every bird can change speeds, but has a top speed during flight because drag doubles with an increase in speed. The fastest bird is the peregrine falcon. Its wings have a swept back design enabling it to fly at 100 mph in level flight. By folding its wings against its body, the falcon can dive at 200 mph.

Vulture Wing	Pheasant Wing

Also, there are differences in the kinds of flight speeds birds need. For example, the grouse, pheasant, or quail spend most of their time on the ground. They are camouflaged for protection, but occasionally the do need to fly quickly to escape a predator. They are able to catapult straight into the air, powered by short, broad wings. Their muscles are designed for short bursts of speed. Upon examination of the breast muscle, the meat is white. This means there are not many blood vessels to supply energy to the muscle for sustained flight, but it does make good eating. Ducks, on the other hand, have red breast muscle because they are capable of long, sustained flight. Most birds do not fly faster or higher than necessary. It takes too much energy to climb against gravity and higher air means less oxygen to breathe and support the bird.

Maneuverability is another variable in flight. Some birds can make sharp turns at top speed, others fly primarily in straight lines. The difference is in the tail design. It is used like a rudder. The tail feathers are broad and stiff and can open and close like a fan, move up or down, or twist left or right.

Birds and planes must bank as they turn. They tilt one wing higher than the other to prevent sliding. If the try and turn without banking, they skid because there is nothing but air to hold them. Their bodies keep going in the former direction even though they are facing in a new direction. Banking braces the underside of the wings and body against the on coming air and reduces the skid. Banking is not the only method for turning. Some birds beat one wing a little faster than the other. They then turn in the direction of the slower moving wing. Steering up and down is accomplished with the tail by moving it opposite the desired direction. Turning while gliding and soaring is accomplished by two different wing designs. Birds that soar over land have broad, slotted wings, while those who soar over water have long, narrow wings. Both are equally good.

Another large variation in flight is found in birds that dive. Penguins have a number of special adaptations . Their wings that are more fin like, but they don't swim like fish. Instead, they use their flipper -like wings to move through the water like other birds use wings to fly, and at almost the same speed! Although flight requires lightness, birds that dive can't be too light. Emperor penguins, which weigh 90 pounds, have been found to carry stones in their stomach, perhaps as a ballast to keep them underwater. Because they eat fish, they do not need stones for digestion. Penguins also have large fat layers for protection against the cold. Because penguins need to see under water, the lenses on their eyes are focused for underwater vision and penguins are very near-sighted on land.

The birds that spend most of their time in the water, but can fly when necessary, have trouble getting airborne. Once up, they are strong, fast fliers. They drag their feet behind while flying and use them to steer. Underwater, their feet act like propellers as they drag behind. Diving birds can stay submerged for as long as 15 minutes. When the bird dives, its heart rate and, therefore, oxygen consumption goes down. The air inside their body lasts much longer.

Sea birds have special needs. They have everything they need at sea, except a place to nest. Therefore, they must return to land occasionally. Finding enough food close to the nesting place is often a problem. Birds like the albatross can be at sea for several years eating fish, drinking salt water, flying on the sea breezes, and resting on the water. They have special nasal glands that remove the salt from their bodies, and heavily oiled, waterproof feathers to keep dry.

Different birds use different fishing techniques. Some skim the water surface for fish others dive. Lightness is a problem; therefore, many birds climb 30 to 100 feet into the air before plunging into the water. They approach the water in different ways. Pelicans fly along until they spot a fish. Then they bank sharply, stall, and fall, letting gravity pull them down. Their wings remain partially open to control speed and direction. They hit the water head first, scoop up the fish in their pouch. Upon returning to the surface, it faces the wind for extra lift, drains the pouch, and takes off. Osprey use the same technique until just before entering the water. Instead, it uses its feet to grab the fish as it disappears in a spray of water, When it reappears, the wings sweep back and forth horizontally so it rises up like a helicopter. When it clears the water, the wings switch to vertical motion and it flies away. Diving birds have a special modification on the ends of their flight feathers. They are notched so they cannot stick together, even though the are wet.

The most unique variation in flight belong to the hummingbird. It can hover for long periods of time. Because of their size, hummingbirds flight techniques are more like those of insects than birds. Their bodies are held upright, rather than horizontal. This means their wings do not move up and down, but sweep back and forth, pushing the air downward instead of backward. Each time the wing changes direction, they also twist 90 degrees, so the air is pushed downward in which ever direction they move. This is like the horizontal rotor of a helicopter. Since their wings produce as much power on the up stroke as on the down stroke, their muscle structure is different than other birds. Both flight muscles are large and comprise 30% of their total body weight.

Nature's perfect flying machine is truly remarkable. Because birds can fly, they are found everywhere on earth. Flight is a very efficient way to travel. However, no matter how hard man tries, he will never be able to duplicate nature's flying machine.

Explore Space ... Not Drugs!
Hear what astronauts have to say about staying drug-free.
Last modified: Wed Jul 2 10:56:21 PDT 1997

Copyright © 1997 by Cislunar Aerospace, Inc. All Rights Reserved.