In-Depth Journey North Lesson
This six-part lesson is designed to teach you the basics of how birds
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.
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?
skeleton: designed for flight
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:
don't grow and develop inside the mothers' bodies. They develop
in eggs outside their mothers' bodies.
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.
don't have bladders. A bird urinates as soon as it has to, getting
rid of the useless weight. (That's why you can't housebreak even
the smartest pet bird.)
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?
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.
Cranes are "designed" for flying.
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:
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.)
tail and wing bones are very short, attached to sometimes
long (but always very light) feathers.
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.
Flight vs. Heron Flight
fly with neck outstretched.
fly with neck in a crook.
3: Many people confuse herons and cranes. Watching how they
fly will give you a clue. Herons pull their neck into a crook
fly, while cranes fly with their neck outstretched. Why do you think
these two families fly in different ways? HINT: think about
foods they eat and how they catch them, and don't forget about their
center of gravity!
In order to fly, birds must do four things:
up in the air
up there as long as they need to
in the direction they want to go
back down safely
wings help them to accomplish all of these jobs. Let's look at each
1. Getting up in the air
Cranes taking off
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.
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.
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!
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
birds get up in the air, they use two main flying techniques to stay
When birds soar, they take advantage air currents to help hold them
up. Three kinds of air currents are especially helpful to soaring
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.
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.
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.
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
Heading in the right direction
a hawk flies from left to right, it spirals up on one thermal and
then glides downward toward the next thermal.
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
with they're facing the wind, but their easiest time moving forward
when being pushed by the wind. But because 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!
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 and geese use their outstretched feet
as well as their open wings to increase drag, acting as brakes to slow
When airplanes are up in the sky, they tuck their landing gear in so
it doesn't slow them down. Most birds do the same thing, by tucking
their feet and legs beneath their tummy feathers. Birds with very long
legs or legs too far back on their bodies can't tuck in their legs.
4: Can you think of some kinds of birds that don't tuck their legs
in as they fly?
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. Their
long, wide wings allow them to fly using different kinds of flight techniques.
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.
silhouette was drawn from a real crane. But can it fly?
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.
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.
Science Education Standards
- Each plant
or animal has different structures that serve different functions in
growth, survival, reproduction.
systems at all levels of organization demonstrate the complementary
nature of structure and function.
have observable properties, including size, weight, shape, color,
temperature, and the ability to react with other substances.
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.
North is pleased to feature this educational adventure made possible
Whooping Crane Eastern Partnership (WCEP).