In-Depth Journey North Lesson
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.
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.
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.
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
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:
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
vs. Heron Flight
fly with neck outstretched
Photo Dr. Glenn Olsen
fly with neck in a crook
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!
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
1. Getting up in the air
cranes taking off.
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.
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
2. Staying up there
birds get up in the air, they use two main flying techniques
to stay up there.
When birds soar, they take advantage air currents to help hold
them up. Three kinds of air currents are especially helpful to
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
- 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.
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.
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 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!
4. Coming down safely
Richard van Heuvelen, Operation Migration
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.
4: What are 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, balancing
each other so their center of gravity is between their wings (where it
needs to be for long
Their long, wide wings allow them to fly using different kinds of flight
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
silhouette was drawn from a real crane. But can it fly?
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
Science Education Standards
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.