An In-Depth Look at Eagle Eyes
pigeon skull shows how large bird eyes really are!
Bird vision has impressed and baffled humans for centuries.
Scientists consider bird eyes to be the finest in the animal kingdom.
And raptors have the finest vision of all. Small wonder just about
everyone knows the expressions "bird's eye view" and
ago, scientists observed eagles fishing, hawks and falcons dive-bombing
prey from great distances, robins cocking their heads before pulling
out a worm, and nighthawks snatching moths out of midair, and figured
these birds must have extraordinary vision. When people examined
dead birds, they noticed that the eyes fill a huge portion of the
head. Bird eyes sometimes even weigh more than the bird's entire
Eyes Have It
It's impossible to know for sure what the world looks like to an
eagle, but we know from studying the anatomy of their eyes that
their view must be enlarged and magnified compared to our view.
Eagle eyes are the same size (weight) as human eyes (though a full
grown adult Bald Eagle weighs no more than about 14 pounds!)
But an eagle eye has a much different shape from ours. The back
is flatter and larger than the back of our eye, giving an eagle
a much larger image than we can see. And its retina has much more
concentrated rod and cone cells-the cells that send sight information
to the brain.
a distant dragonfly might look to an eagle
fovea of an eagle has about a MILLION cones per millimeter.
the same dragonfly might look to a person
human fovea has 200,000 cones per millimeter
Let's assume eagles have exactly 1,000,000 cones per square
millimeter in their central fovea, and humans have exactly
200,000. If this was the only difference between our eyes,
and if the farthest we could clearly see a 3-inch mouse
was 200 feet, what would be the farthest an eagle could
clearly see that same mouse? (>>)
Notice how light passing from air to water makes this ruler seem
bent. This refraction can make it hard for eagles to know
exactly where the fish are in the water. Their eyes don't seem to
have any adaptations to correct for refraction, but their brains
do! The first fish young eagles successfully catch are often dead
ones floating right on the surface of the water. They miss live
prey a lot when they're first learning to fish. Fortunately, with
experience they slowly learn how to correct for refraction.
ruler isn't bent. Light hitting it bends as it passes from air
to water. This
is called refraction.
Look at the little boy's eyes and the Bald Eagle's eyes (The
boy's name is Tommy. We don't know what the eagle's name is.)
eagle has a little bit of bare skin between its eyes and its beak,
and a bony ridge above its eyes. That bony ridge makes its face
appear fierce to us. Look at Tommy's eyebrows, and feel above your
own eye. You have a bony ridge above your eyes, too, but in most
people it's not quite as noticeable as on an eagle, and certainly
doesn't make Tommy look fierce!
do you think people have a bony ridge above their eyes? Why
might this bony ridge be so much more noticeable in eagles?
might the skin right in front of the eagle's eye be bare? (>>)
and the eagle's eyes are wide open! Like you, Tommy has a big top
eyelid with long eyelashes, and a small bottom eyelid with shorter
eyelashes. His lids open and close from top to bottom, but not from
side to side.
Eagles (and other birds) have 3 eyelids! The outside two are the
ones we usually see. On eagles the bottom eyelid is bigger than
the top eyelid, so they blink up instead of down. Birds also have
an inner eyelid, called a nictitating membrane. This eyelid is transparent,
and sweeps across the eye from side to side. It grows in the inner
corner of the eye, right next to the tear duct. Look in your partner's
eye or in a mirror and see if you can see a tiny hole in both the
upper and the lower eyelids, right in the inner corner of your eye.
These are tear ducts. Can you see tissue in the corner of your eye
that is related to a bird's nictitating membrane?
Why do you think birds have a nictitating membrane?
you know that your tear glands are always making tears, even when
you're not sad or peeling onions? Tears help to keep the eye moist,
and have a special chemical called a lysozyme that kills bacteria,
protecting the eyes from infection. Birds have tear glands that
secrete watery tears like ours, and birds that spend a lot of time
in the ocean have another, special kind of gland that secretes oily
tears too, to protect the eyes against salt water. Eagles have these
glands, but they're smaller and not as important for eagles as they
are for cormorants and other ocean birds.
do you think tears are salty? And if we humans and birds are
always making tears, where do they go when we're not crying?
The tiny speck of white in the center of Tommy's eye is just
a reflection from the flash when the picture was taken. Tommy's
irises are so dark brown that it's hard to see his pupils in this
photo. The eagle's irises are pale yellow. The white part of Tommy's
eye, which isn't a seeing part of the eye at all, is called the
sclera. This eagle's eye also has a sclera, but it's hidden under
If there were no skin to hide them, all eyes would appear bigger
and round from the front. But both humans and birds have skin covering
part of the eye. The eyelid openings for human eyes are oval-shaped.
The eyelid openings for bird eyes are round.
Why do humans need oval-shaped eyelid openings to see well? Why
do birds need round-shaped ones?
Tommy is two years old. His eyes are already just about as big as
they're ever going to be! This Bald Eagle's eyes are about the same
size as Tommy's. The eagle's head is smaller than Tommy's, but its
eyes are just as big, or even a little bit bigger.
again at the picture of a bird skull. Bird eyes are MUCH bigger
relative to their head size than human eyes! And their brain is
much smaller. People used to think that meant that birds were stupid
compared to mammals, but now they are learning that birds are more
intelligent than they thought!
of Journaling Questions
An eagle could see the mouse 446 feet away, and can see 2.24 times
better than humans can. This is how we figured out the answer: The LENGTH
of the mouse is 3 inches. That means we can't think about the AREA of
1 millimeter square, but the LENGTH of it. An eagle has 1000 cones along
an edge of that area (the square root of 1,000,000), and a human has
447 (the square root of 200,000). So just considering the fovea, an
eagle could see 2.24 times as far as we can see.
What do you think is a good reason why people have a bony ridge above
their eye? Why might this bony ridge be so much more noticeable in eagles?
Answer: The bony ridge above human and eagle eyes does two jobs:
protects the eyes from blows and helps shade the eyes from sunlight.
Our skulls AND eagle skulls have a fairly similar job of protecting
from physical injuries. But eagles, sitting at the tops of trees or
fishing in the open on lakes and rivers, need more protection than we
do to keep the sun out of their eyes. Humans can stay in the shade on
bright days, and our eyebrows help protect our eyes so they don't need
as big of a bony ridge for protection. Plus, WE can wear sunglasses.
might the skin right in front of the eagle's eye be bare?
The only covering on skin that birds have is feathers. If even tiny
feathers grew in front of an eagle's eye, they might block the view,
get caught in the eye, or brush against it, especially when the eagle
was flying. This would make it hard to see and maybe even scratch the
do you think tears are salty? And if we humans and birds are always
making tears, where do they go when we're not crying?
Our tears are salty because they come from body tissues, and our
bodies (our blood and tissues) are just as salty! Our tears drain into
the nasolacrimal duct, which empties into the nasal cavity. No wonder
when we cry our noses get snuffly! Eagles don't produce as many tears
as we humans do, and they are constantly being swept across the eye
by the nictitating membrane, so eagles don't get snuffly. Lucky, too,
because they don't have a nose to blow.
An In-Depth Look at Eagle Eyes - Part
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.