Introduction

Every time we look into the sky we are fascinated by birds flying through the sky. From the very start of recorded time we have been watching birds fly. They do such wonderful things: taking off, flying with twists and turns, soaring and diving, and landing again on a small branch.

Fossil records show that birds have been flying for millions of years. It has only been within the last hundred years that we have started to understand the intricacies of flying. No matter how hard we tried to fly, man had to unravel the mysteries of flight.

Man had to first be able to see the forces on earth that enable flight, and then try to imitate a bird's actions. It was not until man developed the internal combustion engine that he could deal with the problem of minimum weight and maximum power. And with the invention of the slow motion camera we were able to view the split second movement of a bird's wing in flight.

internal combustion engine

Now we could understand what we could not before. Now we would be able to conquer controlled flight. A bird is a perfectly controlled, natural flying machine. The great diversity with beautiful colors, songs, and habits makes each species unique. It is the great identities among birds that enable scientists to truly understand the combinations each species possess. Everything about a bird is designed for flight.

In order to understand the natural flight of birds, we must first look at the structures of a bird. What makes a bird a bird? They all have common characteristics. We will explore the earliest birds. We will look at the actual mechanics of flying to understand how birds take off, fly, and land.

What Makes a Bird a Bird?

Birds are unique. In the animal kingdom birds have two reasons for being special. 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 have a backbone. Birds are warm blooded. This means that their body temperature remains the same even in differing temperatures.

Birds lay eggs and defend themselves with a bill or a beak. All birds have wings, but not all birds fly. Birds are the most varied and most successful of the vertebrates. There are 8,600 species of birds in the world today. They are found everywhere. Each species has its own range: determined by oceans, climate, available foods and nesting places. No one bird is found everywhere. But every bird has special adaptations to fit their unique lifestyle and niche

Birds play a vital role in the balance of nature. They eat insects, pests and small animals. Fruit eating birds are best for scattering seeds for these plants. Hummingbirds help pollinate nectar producing flowers. Seed eating birds digest seeds and in so doing keep millions of weeds from the earth.

Birds also have other values. We eat both eggs and meat from birds. These are sources of protein. Feathers are used for pillows, quilts and clothing. Down feathers are especially good insulators. Also, many people keep birds as pets.

Man is in awe of the beauty of the bird and has long been an inspiration for him. People have written poetry, stories, and songs about birds. Birds have also become symbols for human values. Owls mean wisdom, the dove peace, and the eagle political power. The bright, colorful feathers of birds have been used for ornaments and clothing for 100's of years.

Physical Characteristics of a Bird

The scientific study of birds is called ornithology. This type of study started in the 1700's. Using direct observation many physical characteristics of a bird are noticeable.

Feathers

The first thing that we notice about a bird is its feathers. Feathers are designed in such perfection. They are light but very strong, and they are flexible but very tough. Beaks and eyes have no feathers and most birds have featherless legs and feet. Feathers do not grow all over the bird. Scavenger birds have very few or no feathers on their heads and necks because they would get dirty when they eat.

feather

The entire body of the bird appears to be covered with feathers. But this is not true. Feathers grow in certain areas called feather tracks. The contour feathers overlap so they appear solid. Limiting the number of feathers helps to keep down the body weight. In between the feather tracks are down feathers.

Why are feathers light? Feathers are made of a tough and flexible material called "keratin". Since feathers are not living, they do not need blood. Growth begins with the tip of the feather. As an embryo the tip grows out toward the skin and then breaks through. When a bird hatches the tip separates and appears as fuzz on the baby bird. This is soft down. It is not a feather but a covering.

Feathers look solid, but they are not. The spine down the middle, called the shaft, is hollow. The vanes are on the two halves of the feather. They are made of thousands of branches called barbs. Because there are many spaces between these barbs, a feather has as much air as matter.

The tip forms a tough, protective sheath. 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 if 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 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. Birds take a lot of time caring for their feathers. This process is called preening. They use their feet and beaks to arrange their feathers to keep them smooth. Birds also bathe alot because dirt adds weight. Most birds have an oil gland at the base of their tail. Frequent oiling keeps the feathers water-resistant and shiny.

Feathers wear out and need to be replaced. The process of loosing old feathers and growing new ones is called molting. This happens in most birds once or twice a year. Feathers are lost in an orderly fashion and at various rates, depending upon the species. Most birds take about 2 months to molt.

Birds have between 1,000 and 25,000 feathers. Larger birds have more feathers. The swan with its long neck has the most. Feathers can be divided into 6 categories:

1. Contour feathers are the most abundant and cover the outer surface of the bird.
2. Semiplume feathers are shaped like the contour feathers. Its shaft is not stiff and its barbs have no hooks so are fluffy.
3. Down feathers are like semiplume feathers but have a very short shaft. They keep the bird warm.
4. Filoplume feathers are tiny and delicate with only a few barbs on the tip. They are scattered sparsely among the other feathers.
5. Bristle feathers are stiff, hairlike feathers found in only some birds. Their function is specific in each species. For example, they cover the nostrils of woodpeckers and in ostriches form eyelashes.
6. Powder-down feathers grow continually. This feather is only found in a few birds. The metallic sheen of the heron is caused by this powder down.

Not all birds have all the types of feathers. It depends on what type of bird it is. But all birds have feathers on their wings. The bird's wing is the basic structure for flight. It is the shape of the wing that allows a bird to fly. The shape of the wing is made by the feathers.

Although the curved airfoil of the wing is necessary to lift the bird into the air, feathers help 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 feathers bends upward, pushing down the leading edge. Each feather performs like a propeller with every wing stroke.

In order to prevent stalling, each flight feather has a notch on the leading edge. The narrowing of the vane leaves spaces between the feathers when the wing is spread which helps even out the airflow.

The tail of the bird plays a big role during flight. The tail acts as the rudder, balancing and steering the bird. The tail also helps the bird in stopping. The tail is turned downward and acts like a brake. 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.

Feathers are truly amazing. They protect the bird's skin and insulate him. Feathers can be fluffed up in the winter or squeezed down in the summer. Feathers are also used to line nests. Owls have densely packed face feathers which help the owl gather and focus sound. In this way they can hunt in total darkness.

The color of the feathers is important in mating. Because birds fly they don't need as much camouflage and can have much more colorful skin coverings. Other mammals cannot. Birds can see color. Other mammals cannot. The amount of color found in birds is dependent upon their lifestyle. Feathers truly make birds unique in the animal kingdom. The secrets found in a bird's feather is awesome.

Beaks and Feet

The beak or bill of a bird is another unique feature. A bird does not have a heavy jaw bone and teeth, but a lightweight beak. The shape of a bird's beak varies with the type of food it eats. 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.

bird beaks

All birds have 2 legs and 2 feet. On the ground most birds get around by walking, hopping or climbing. The shape of the feet and legs is different for each type of bird. It is dependent upon their use and function. All birds have claws on their toes. But the birds of prey have the most pronounced claws.

bird feet

Internal Structures of a Bird

Flight is much more than feathers and wings. Everything about a bird is made perfectly for flight. Their entire body inside and out has been designed for flight. This includes their skeletal, muscular, nervous, circulatory, respiratory, digestive, and reproductive systems. The key factor in adaptations for flight is lightness.

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. The biggest flying bird today is called the Great Bustard of Europe and Australia. They weigh as much as 32 pounds and are 4 feet long.

The largest bird ever to fly is extinct, but its bones were found in Nevada. Its scientific name is "Teretornis incredibilis". Its wing span was 17 feet.

The extinct Moa of New Zealand stood 12 feet and weighed 700 pounds. And the elephant bird weighed 900 pounds. Scientists guess that these birds grew so big that they did not need to fly because there was plenty of food on the ground.

The best way to fly is to be light. So most birds are small. Birds have feathers that are very light weight. The bones that birds have are also made for lightness. Birds have less bones than most animals. The bones they have are hard but thin. The biggest bones in flying birds are the breast bone and the shoulder bones.

Some birds have bones weighing less than their feathers. The powerful flying birds have medium weight skeletons. Birds that don't fly have the heaviest skeleton. In birds many of their bones are fused together. In this way not as many muscles are needed. The biggest muscles a bird has are its flight muscles. These are very strong because they have to raise the entire body into the air.

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. Not all muscles in birds are used for flight. Most birds also walk, hop, or swim. So they also have well developed leg muscles. The structure of a bird's eyes is very special. Birds have 3 eyelids. There is an upper lid like humans. 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 also protects 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. This gives them the same effect as a zoom lens on a camera.

The actual image a bird sees depends on the placement of the eyes. The eyeballs of birds are flatter than those of humans. And the retina of a bird's eye is 2 times as thick as a man's. A sparrow hawk can see 8 times more clearly than a person.

Hearing is another sense that is strong in birds. The ears not only hear well but are used for maintaining balance. This is critical during flight. The muscular control area of the brain is also well developed. This is because flying is such a complicated series of muscular movements

Internal Power Systems

Getting into the air requires a great deal of energy and taking off requires far more than moving forward. Also, birds must have extra power to maintain a constant body temperature. A large bird burns extra energy taking off and a small bird uses extra energy keeping warm. So they both need to be well designed for supplying energy to their bodies, while maintaining lightness.

Birds have a natural way to solve this power problem. All of the internal organs of a bird run at high speed. This makes for high energy, but also shortens the life span of the bird. Some birds only live 2 years. The food that the birds eat must be light weight and "high-octane". These foods are high in calories. They are seeds, nuts, fruits, fish, and rodents.

The speed at which a hummingbird burns its food is 50 times greater than man. At night a bird must rest and so their body functions slow down. A hummingbird would starve to death at night if his heart, respiration, and body temperature did not slow down.

When the food enters the stomach of the bird, digestion begins. Very strong gastric juices begin the process. The lower part of the stomach, the gizzard, has powerful muscles for mixing and crashing. Birds that eat hard grains and seeds must swallow small stones or shells to help the gizzard grind the food. The digestive process is very rapid - 20 minutes. Birds are able to "eat and run".

To create energy from the foods they eat, birds must burn them up. This is called combustion and oxygen is needed to do this. Oxygen is taken into the body through the lungs where it passes into the bloodstream. Birds need a very fast system to supply the energy they need. In birds also there are air sacs next to the lungs. They never run out of breath.

The lungs of birds work faster than ours. In the birds' lungs, the air moves continually in one direction throughout the system of tubes and air sacs. These tubes are open at both ends so that stale air can be flushed out easily. There is always fresh air in the lungs.

A bird's heart beats much faster than other animals. This makes the blood travel through a bird in seconds. Because of this fast blood travel, a bird's body temperature is 7 or 8 degrees higher than a persons. 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. This is like what a dog does when he pants. But during flight, the lung surfaces are not large enough to expel all the extra heat. So the large air sacs must be used also.

The Mechanics of Flight in Birds

The distinctive shape of a wing is called 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.

wing diagram

The air on the upper part has a lower pressure than the air moving over the lower part. This pressure difference makes the lifting possible by the wing. There is another way to get the most lift. The bird can make "slots". Birds can move their feathers at their wing tips to produce these slots.

Speed is the most important part in making lift. Speed can be greater by increasing the forward speed of the wing itself as it travels through the air. If you double the speed, the bird gets 4 times the lift. If the bird triples the speed it will get 9 times the lift.

The bird makes its speed by flapping its wings. This forward motion is called thrust. A bird's wing is like the arm of a man. It is the "hand" part of the wing that makes the power to move the bird through the air. The structure of the bird's wing can be folded close to its body when it is not in flight. But when the wing is stretched out it is both the wing and propeller for the bird. The feathers attached to the "hand" bones are the ones that make the forward thrust for the bird.

Try moving your arms really fast. Then turn your arms as you move them. More air motion happens as you twist your arms. This is how it works with a bird. The twisting is automatic with the bird and caused by the unequal size of the vanes. 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. This causes the twisting of the feather in the opposite direction. So, as the wing flaps up and down, the feathers twist back and forth.

The bird does this motion in an instant. It took man a long time to find out this secret. When man could see this motion with a slow motion camera it helped us alot.

The secondary feathers are also rotated into a closed position. This causes the entire undersurface to push down and backward against the air, helping to move the bird upward and forward. 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.

For most birds the upstroke is only a means for returning the wings to the starting position for the next downstroke. At the top of the upstroke the wings are once again positioned for the next downstroke. The flight feathers are still open. When the large breast muscle pulls, the feathers close and the cycle repeats itself over and over. This single wing beat takes only an instant to occur.

Gliding Flight

Some birds can glide and soar for hours. Eagles and vultures glide for miles. How do they do this? If the power does not come from their wings, where does it come from?

Birds take advantage of upward movements of air called thermals. These birds are resting on piles of air that are moving upward. Many birds find these thermals and use them for the upward push they give. These birds are gliding downward on a constantly rising current of air.

vulture

Warm air does not rise from the ground in an even manner. This air is shaped more like doughnuts piled up on one another. Riding thermals are great energy savers for birds when they can find them. Eventually the thermal thins out and cannot push the bird upward. When this happens the birds glide down until they can catch another rising current of air.

Thermals do not always exist. The sun has to heat the ground enough for thermals to happen. Many large birds wait on the ground until 10 o'clock in the morning for the thermals to form.

There are other kinds of updrafts also. Obstruction currents are made when moving air runs into an object like a cliff, mountain, or tall building. The air is forced up and over the object. Birds can "ride" these currents. Birds by the ocean can also glide and soar using trade winds. These winds give a source of power which the birds can glide on. This is called dynamic soaring.

Take Off and Landing

The time in any flight of the greatest danger is during take off and landing. This is because speed is related to the lift needed to leave the ground. Lift is the weakest at take off because full speed has not been made. And at landing because the speed of the bird is being reduced.

The size of the wing is what determines how much lift is made. The angle at which the wing approaches the air affects lift. This increases the air speed over the wing and produces more lift.

Birds use all of these aids during takeoff:

1. Some birds rotate their wings at the shoulders.
2. Some birds change their wing size by twisting their primary feathers.
3. Some birds use the wind.
4. Some birds flap their wings faster.
5. Some birds use their long legs to push up into the air.
6. Other birds run along the water to start off.

Larger birds have a harder time getting into the air. They need a strong wind or they land on cliffs. The South American condor, with a wing span 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 a cliff. From the cliff they can leap off to begin their next flight.

The funniest takeoff and landing is made by the "Gooney Bird". They run, flap, and hop. With the head wind they make it into the air. To land the gooney bird "starts falling". It crash lands on its nose!

Landing is harder than takeoff. The bird must end his flight slowly. First, the bird slows its wing beats. Next, gravity begins to pull the bird down. Then the bird twists its wing to increase the surface area of the wing. This makes more lift to slow the birds landing.

At the same time, the whole wing is moved slightly at the shoulders to increase the angle of landing. This makes more lift as the speed is slowed. This prevents the bird from dropping too fast. And last, the tail is spread open and lowered to act as a brake.

The bird may approach the target too quickly. If this happens it can put its propellers in reverse. By twisting its wings slightly at the shoulders air pressure automatically bends the outer primary feathers at the tips. Now when the wings swing forward the feathers push up on the air, in the opposite direction that they normally do. This acts as another brake in the forward motion.

The moment of impact can still be dangerous. The branch may be moved by the wind or a gust of wind may roll the bird. If this happens the bird must flap its wing once again to correct the situation. The feet finally grab onto the branch and the strong leg muscles absorb the final shock. The flight has ended.

Variety in Flight

Birds live all over the earth. The diversity of habitats gives the different types of birds a sanctuary made for them to meet their needs. There are many types of birds. Some are large and some are small. The top speed of a bird depends on its design. The vulture flaps its wing once per second. A small bird flaps its wings 4 times per second. The hummingbird flaps its wings 10 times per second and flies at 60 mph. Every bird can change their speed, but has a top speed during flight.

peregrine falcon

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.

Some birds do not fly very much. For example, the grouse, pheasant, or quail spend most of their time on the ground. These birds are camouflaged for protection but occasionally need to fly away quickly. They are able to fly straight into the air, powered by short, broad wings. Their muscles are designed for short bursts of speed.

Most birds do not fly faster or higher than is necessary. This is because it takes too much energy to climb against the gravity. Also, there is less oxygen to breathe and support the bird high in the air. Being able to move quickly in the air is another benefit that many birds enjoy. Some birds make sharp turns at top speeds. Other birds fly mostly in straight lines. The difference is in the tail design. Because the tail is used like a rudder the feathers are broad and stiff. These tail feathers open and close like a fan and move up and down. They also twist to the left or right.

Birds "bank" as they turn. They tilt one wing higher than the other. Banking holds the underside of the wings. Another way the bird turns is to beat one wing a little faster than the other. Birds that soar over land have wings that are broad and slotted. And birds that soar over the water have long and narrow wings. Both are great.

Another variation in flight is found with birds that dive. These birds have special wings. Penguins have flipper-like wings. They use these to move through the water. Although flight requires lightness, birds that dive cannot be too light.

Emperor penguins weigh 90 pounds. They have been found carrying stones in their stomach. We believe this acts as an anchor to help them stay underwater. Underwater birds that dive can stay under the water for 15 minutes.

Birds use many ways to fish. Some skim the water surface for fish. Others climb 100 feet into the air before diving into the water. For example, pelicans fly along until they spot a fish. They bank sharply, stall, and fall, letting gravity pull them down. 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. The Osprey use the same techniques as the pelicans except it uses its feet to grab the fish.

Diving birds have special ends on their flight feathers. They are notched so that they cannot stick together, even though they are wet.

The most unique flying bird is the hummingbird. It can hover in one place for long periods of time. Their bodies are upright instead of horizontal. This means that their wings do not move up and down. They sweep back and forth pushing the air downward instead of backward.

Each time the wing changes direction the hummingbird also twists 90 degrees. 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 make up 30% of their total body weight.

The beauty and function of birds is beyond our ability. They are natures perfect flying machine. Because birds fly they are found everywhere on the earth. Flying is such a wonderful way to get around. We try to imitate the bird. But, however hard we try we cannot. Man can't do what a bird does naturally.

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Last modified: Fri Jul 10 14:55:34 PDT 1998

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