How Birds Fly
An In-Depth Journey North Lesson

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This six-part lesson is designed to teach you the basics of how birds fly.

Introduction
Birds have beautiful feathers and lovely songs that bring joy and wonder to us humans. And flight is the feature that probably captures the human imagination more than anything else. For millennia, people have watched birds in the sky and wished we could fly, too.

There are almost as many ways of flying as there are kinds of birds. Albatrosses glide and soar with long narrow wings stretched out, sometimes staying aloft for hours without a single wing beat. Hummingbirds, on the other hand, can't rest their wings for even a second in flight. Woodpeckers have a swooping flight, crows fly in a straight line, and swallows dart and weave every which way.


The Gravity of the Situation
Isaac Newton is the scientist who first realized that gravity is a force between two objects that draws them toward each other. The more mass an object has, the more it pulls other things toward it. The largest object anywhere on earth is the planet itself, so gravity pulls everything down toward the center of the earth.

Question 1: If gravity pulls everything down, why do helium balloons go up?

Bird skeleton: designed for flight
a bird's skeleton
Gravity pulls on birds, too. In order to minimize the effects of gravity, birds are adapted to be as light as possible. These are some adaptations that help make birds light:
  • Hollow bones
  • Feathers
  • Babies don't grow and develop inside the mothers' bodies. They develop in eggs outside their mothers' bodies.
  • Birds eat foods that are very high in usable calories so they get as many calories as possible from from a small amount of food. Seeds, fruits, and meat (from prey) are the main food items for birds. Virtually no birds (except the Hoatzin, which lives in South America) eat leaves, which take a long time to digest. Their efficient digestion allows birds to get rid of useless weight very quickly.
  • Birds don't have bladders. A bird urinates as soon as it has to, getting rid of the useless weight.


Winging It
Birds are not the only animals that fly. A huge number of insects fly and so do a few vertebrates. Flying fish and flying squirrels can take off and glide through the air for fairly long distances, and bats are very well adapted for genuine flight. But there are not nearly as many kinds of bats in the world as there are birds.

Question 2: Why might bird wings be more adaptable to flight than bat wings?

Bird wing bones
the bones and muscles of a bird's wing
A bird's breastbone, or sternum, is shaped like a keel to attach the powerful wing muscles. The bones of a bird's wings are surprisingly small compared to the size of the wing. All the bones and muscles of the wing are in the front and covered with feathers that protect and streamline the wing. The actual flight feathers are attached to the wing within little pits in the bones.


Designed for Flight
Whooping Cranes are "designed" for flying
Bird wings are not the only part of their bodies designed for flight. Just about every part of a bird body is specially adapted to help the bird fly. A bird's center of gravity is the balance point between its two wings and between its head and tail. If it were possible to perfectly support a bird right at its center of gravity without it squirming around, the bird wouldn't tip in any direction. To fly well, birds must have most of their weight in their center of gravity, and very little weight in front of or behind it. Their bodies have many special adaptations to help accomplish this. A few are described here:
  • Birds don't have teeth or a nose, which are heavy and would be too far forward. To grind their food, their stomachs have a gizzard near their center of gravity. They use their mouth and the nostrils located on the top of their lightweight beak to breathe. (Their nostrils are also used for smelling. Older bird books say most birds can't smell, but current research proves that many birds have at least some sense of smell.)
  • Their tail and wing bones are very short, attached to sometimes long (but always very light) feathers.
  • Bird lungs don't fill up with a lot of air like ours do. All vertebrate lungs (including birds') need to be placed near the heart. Our huge, lightweight lungs set in our chest work fine for us, but birds need their heaviest organs in their chests. So their lungs, which can hold very little air, are flat and sit against their back ribs. The air birds' breathe in flows through the lungs into big balloon-like air sacs that fill much of their lower abdomen, behind their center of gravity. When they breathe out, the air flows back through the lungs through different passages. Their lungs are VERY efficient at pulling out oxygen (which they need in great quantity) as streams of air go in and out.
Crane vs. Heron Flight

Heron taking off

Cranes fly with neck outstretched
Photo Dr. Glenn Olsen

Herons fly with neck in a crook
Photo Brian Small

Question 3: Many people confuse herons and cranes. Watch how they fly and you will see clues: Herons pull their neck into a crook while they fly. Cranes fly with their neck outstretched. Why do you think these two bird families fly in different ways? HINT: think about the different foods they eat and how they catch them, and don't forget their center of gravity!



An Aerodynamics Primer
In order to fly, birds must do four things:
      1. get up in the air
      2. stay up there as long as they need to
      3. move in the direction they want to go
      4. come back down safely

Their wings help them to accomplish all of these jobs. Let's look at each job:

1. Getting up in the air

Whooping cranes taking off.
Whooping Cranes coming in for a landing
Birds have many different ways of taking off. Some, like loons, run into the wind, and the rush of air beneath their wings lifts them up. Others, like puffins and Peregrine Falcons, jump off cliffs and other high perches. Chimney Swifts simply let go of their chimney or other vertical perch, and fall into the air. Hummingbird wingbeats are so powerful that they can go straight up from a perched position without jumping. Songbirds, cranes, and many other species leap up on strong legs while flapping their wings, and there they go.

As the airfoil moves to the right, the air above it, going a longer distance, must travel faster than the air below it. This makes the pressure above lower than the pressure below, creating lift.
airfoil
We humans could try leaping and flapping our arms, or running into a stiff wind, but we wouldn't get very far off the ground! The reason birds can is because of the special shape of their wings. The bones of bird wing are in front, covered with a smooth layer of feathers that taper toward the back. The back of the wing is just a single layer of flight feathers. People who study aerodynamics say a wing has this shape to serve as an airfoil. When air comes straight toward an airfoil (from facing directly into the wind or running fast into the air) the special shape causes the air to flow faster on top of the wing than under it. The faster air above lowers the pressure (sort of sucking the bird up) while the slower air below raises the pressure (pushing the bird up). These forces raising the bird are called LIFT, which makes the bird go up!

Blowing above this paper creates lift

Try This!
To see how an airfoil works, hold a narrow strip of paper near your mouth and blow across the top. The air moves faster above than below, and the paper will rise. Does this work with a larger piece of paper? Why or why not?

2. Staying up there

Once birds get up in the air, they use two main flying techniques to stay up there.

Soaring: When birds soar, they take advantage air currents to help hold them up. Three kinds of air currents are especially helpful to soaring birds.

  • Thermal air currents develop in places where the air is warmer in one spot than an adjoining area, such as a paved road alongside a snowy field. Even on a very cold day, the sun will heat the pavement at least a few degrees more than the snow. This slightly warmer air is slightly lighter than the colder air, and rises. This rising air current can lift very light objects, like feathers and hollow bones. The birds that most often take advantage of thermals (like the hawks that fly along coastlines) usually have very wide wings and tail. This makes the area of their wings very large compared to their body weight.
  • Updrafts, also called obstruction currents, develop when wind hits an obstruction, like a cliff or a building. The rushing air has to go somewhere, so it goes up, and can carry a bird up with it. Birds who fly on updrafts (like the many hawks that migrate along Hawk Mountain, Pennsylvania) also have very wide wings and tail.
  • Wind moving toward a bird with spread wings can hold the bird up, thanks to the airfoil shape of the wings (see airfoil illustration above). Birds that fly on moving air currents often have long, narrow wings, such as gulls and albatrosses.
crane02WCEP_131
Photo Operation Migration
Flapping: When birds flap, the stroke of their downbeat moves the wing tips forward and downward. The wingtips make a loop at the bottom of the downstroke, and as the wings move up, the wing tips move upward and backward. In the downstroke, the pressure is higher below the wing than above, causing lift. And as they move forward, the rush of air on their airfoil wings causes more lift. But because flapping birds have smaller wings than soaring birds, they must move forward faster to stay in the air. Most songbirds must fly at least 11 miles per hour to stay up. One scientist calculated that for an ostrich to stay aloft, it would have to take off and maintain a minimum speed of 100 miles per hour. Birds who use their wings to flap more than soar often have smaller wings than soaring birds.

3. Heading in the right direction

When a hawk flies from left to right, it spirals up on one thermal and then glides downward toward the next thermal.
Flight pattern when soaring and gliding
Soaring birds take advantage of thermals and updrafts by flying in a circle. The rising air carries them higher and higher in a spiral. They couldn't simply hold still and go straight up because without moving forward on their airfoil wings, they would simply drop to the ground. But the problem with circling is they don't go in any special direction. So when migrating birds soar on a thermal, they rise as high as the thermal will carry them with their wings spread, and then they pull back the wings into a more narrow point and glide in the direction they want to move. Gliding birds move exactly the way paper airplanes do, slowly losing altitude. So as migrating birds glide, they seek out another thermal to gain altitude again.

Soaring birds that wish to stay aloft without flapping in normal wind usually fly INTO the wind for lift. But that same wind that holds them up slows their forward movements. In order to get somewhere, soaring birds make delicate adjustments to turn slightly now and then. They gain lift for a while and then lose altitude as they head where they actually want, and then gain lift again. This is why gulls usually fly in a more zig-zaggy pattern than many other birds.

Like soaring birds, flapping birds have their easiest time staying up when they're facing the wind, but their easiest time moving forward when being pushed by the wind. Since their forward momentum and the lift they get from flapping is more important than the lift they get from the wind or air currents, they can get where they want to if they just point themselves in the right direction and go!

Photo Richard van Heuvelen, Operation Migration
4. Coming down safely
Many birds, like chickadees and robins, can fly fast until the last seconds and still land easily and safely. To slow down quickly, they change the angle of their wing to be higher and higher, increasing drag (to slow their forward movement) and decreasing lift (to help them move downward). Some birds need to slow down for a longer time in order to make a safe landing. Many ducks, geese, and cranes use their outstretched feet as well as their open wings to increase drag, acting as brakes to slow them.

When airplanes are in the sky, pilots tuck in the landing gear so it doesn't slow them down. Most birds do the same thing; they tuck their feet and legs beneath their tummy feathers. Birds with very long legs or legs too far back on their bodies don't normally tuck in their legs.

Question 4: What are some kinds of birds that don't tuck their legs in as they fly?



Putting It All Together: How Cranes Fly

Cranes have large wings, a long neck, and long legs. They fly with their legs stretched out behind and their neck stretched out ahead, balancing each other so their center of gravity is between their wings (where it needs to be for long flights). Their long, wide wings allow them to fly using different kinds of flight techniques.

When cranes are flying just a few miles or less, they use typical flapping flight. They usually flap with steady beats until they come in for a landing. Then they use their legs and wings to slow down and ease their way to the ground. When cranes are flying long distances, especially on migration, they often soar on thermals until they reach a great altitude, and then use a combination of gliding/soaring and flapping to cover the longest distance using the smallest amount of energy.

Cranes can bend their legs and draw their feet in to their bodies when it's severely cold during migration, but that's exceptional.

This silhouette was drawn from a real crane. But can it fly?
Crane silhouette pattern

Try This!
See if you can design a crane that can really fly, or at least glide. Use cardboard, paper, paste or glue, paper clips, and any other materials you want to try. If you want a pattern designed from a real crane silhouette, click on the small pattern to see a larger sized one. Or try to develop your own pattern, from paper airplane designs or anything else that might work. Test your birds to see which stay aloft the longest, and which fly the farthest.

 


National Science Education Standards

Life Science

  • Each plant or animal has different structures that serve different functions in growth, survival, reproduction.
  • Living systems at all levels of organization demonstrate the complementary nature of structure and function.

Physical Science

  • Objects have observable properties, including size, weight, shape, color, temperature, and the ability to react with other substances.
  • If more than one force acts on an object along a straight line, then the forces will reinforce or cancel one another, depending on their direction and magnitude.



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