An In-Depth Look at Eagle Eyes
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 "eagle eyes"!
skull shows how large bird eyes really are!
Long 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 brain!
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. Some animals, including humans,
have a special area on their retina called the fovea where there is an enormous
concentration of these vision cells. In a human, the fovea has 200,000 cones
per millimeter, giving us wonderful vision. In the central fovea of an eagle
there are about a MILLION cones per millimeter. That's about the same number
of visual cells as the finest computer monitor has on its entire screen when
set at its highest resolution. The resolution for a person would be similar to
setting a computer's screen at a much lower resolution.
how much clearer an eagle's view of a distant dragonfly would be compared
to a human's view of the same dragonfly, if the fovea were the only
difference between our eyes:
vs. Human Vision
a distant dragonfly might look to an eagle
the same dragonfly might look to a person
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?
ruler isn't bent. Light hitting it bends as it passes from air to water.
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.
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.)
The 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!
- Why do you
think people have a bony ridge above their eyes? Why might this bony ridge
be so much more noticeable in eagles? (Link
- Why might the
skin right in front of the eagle's eye be bare? (Link
Tommy's 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
Why do you think birds have a nictitating membrane?
Did 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.
- Why 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? (Link
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 the eyelid.
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
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
Look 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!
What lies beneath
In 1578, a writer named Guillaume de Salluste, Siegneur Du Bartas described eyes
as, "these lovely lamps, these windows of the soul."
Eyes DO work like windows, opening to all the beautiful sights in the world outside
our bodies, from the tiniest hairs growing out of our own skin to enormous stars
so far away that it takes thousands of years for their light to reach the earth.
For humans and birds both, much of the information that we perceive about the
world is processed by eyes.
This diagram of an eagle eye and a human eye shows them as cross-sections, as
if looking down on them from above the head. Look at your own eye in a mirror
or look at one of your classmate's eyes.
- Click on the diagram
of the human eye so you can see the large, labeled picture, and compare
it to your eye or a classmate's eye. Which of the labeled parts can you
actually see on a real eye? Which layers of the eye does light pass through
to reach the retina? Why does the pupil of a real eye look so black? (Link
- If your class
ever has an opportunity to dissect an animal (fetal pig, frog, cat, pigeon,
or something else) make sure you get a good look at the eye!
an open and shut case
Your iris and a Bald
Eagle's iris may be different colors, but they have the same job: to control
the amount of light that shines onto your retina. There are two kinds of muscles
in the iris. Circular muscles encircle the iris close to the pupil, and straight
muscles radiate out like rays of the sun. When the inner, circular ones contract,
the iris gets bigger, making the pupil smaller. When the outer, radiating ones
contract, the iris gets smaller, making the pupil bigger.
Work with a partner. Take turns watching your partner's pupil and iris change
as the amount of light changes.
- Close your
eyes for one minute, then open them while your partner watches.
- Go into a room
without windows, like a closet. If the room doesn't have a light switch,
bring a flashlight. Keep your eyes open in the dark for one minute. Then
turn on the light or shine the flashlight on your eyes for just a few seconds
while your partner watches.
- In a partly
lit room, shine the flashlight close to your eyes (but not directly into
them) while your partner watches. After 30 seconds, turn the flashlight
off while your partner watches.
- In a dimly
lit room, put a hand between your eyes and shine a light on one eye while
your partner watches for differences between your two eyes.
- Which muscles
(the circular ones or the radiating ones) work when the light suddenly
gets brighter? Which work when the light suddenly gets dimmer? (Link
Cornea: window to the world
The first lens that light passes through into the eye is the cornea. This is
a clear window with a curve that we describe as "convex."
When light passes
through any curved lens, it bends. The bending of light through a convex
lens like the cornea makes it "converge." The image formed by the
cornea is upside down and reversed from right to left.
If the cornea were the only curved "window" that light passed through
in the eye, far objects would focus very easily, but near objects would not.
A human's cornea can't change its shape in order to bring objects into focus,
but fortunately, one part of our eye CAN change shape. In order to help us
focus on close objects, the LENS of our eye changes shape. This is called accommodation.
Tiny fibers called ligaments and muscles change the shape of the lens, making
it thinner to focus on far objects or thicker to focus on near objects.
Eagles can change the shape of their lens, and can also change the shape of
their corneas. This allows them more precise focusing and accommodation than
we humans can get.
where vision happens
The retina is where vision actually takes place. Every single thing we see
is projected, upside down and backward, on our retina, onto special cells called rods and cones.
Our human eyes have millions of rods and cones; an eagle's eyes have tens or
hundreds of millions. Each microscopic cone cell is connected to a nerve that
goes straight to the brain. When a tiny particle of light from an object hits
a particular cone, the brain instantly sees it as a particle of color. All
the cones together work like the tiny dots on your computer screen. Your brain
flips the image and puts all the dots together to tell you exactly what you're
seeing the moment you see it.
How an image
appears in the human eye
To see how you would appear in a retina, look at your reflection
in a spoon. You'll be upside down and backward!
Rod cells don't
see color; they simply see light. And several rod cells network with each
other, sending the brain messages on a single nerve. So vision with the rod
cells isn't as precise, but is very fast. Rod cells may see only black and
white, but they are extremely sensitive to light, so they help us see in
the dark and notice quick movements. Eagles have a higher percentage of cone
cells than we humans do, so they can't see as well as us at night, even if
they do see better in daylight.
If a human eye
is shaped exactly right, things focus precisely on the retina. Sometimes
the eye is longer than it should be, and the picture focuses in front of
the retina. This condition is called "myopia" or nearsightedness.
If the eye is shorter than it should be, the picture focuses behind the retina.
This is called "hyperopia," or farsightedness. People wear glasses
or contact lenses with exactly the right curve to move the focus onto the
Eagles with eyes
that are shaped wrong can't wear glasses. Since good vision is so critical
to their ability to get food, eagles with less than perfect vision quickly
starve, and never get old enough to reproduce. So eagle parents all have
great vision, and luckily their babies take after them!
Fovea: Magnifying the view
Some lucky vertebrates (including us humans and just about all birds) have
a special area on the retina called a fovea, where rod and cone cells are extraordinarily
densely packed. As we noted above, a human's fovea has about 200,000 cone cells
per square millimeter, and an eagle's central fovea has over a million. Plus,
certain birds that have especially good vision, including eagles, have a second
fovea. Some scientists consider a long, narrow ribbon-shaped area that connects
the two eagle fovea to actually be a third fovea!
We at Journey North wondered exactly what things look like to an eagle compared
with how they look to us. There is no way to be sure! But we took into account
the difference between the number of cone cells in the central fovea and the
difference in the shape of the eye to make these images of a squirrel at a
backyard bird feeder.
may be how this squirrel would look to an eagle
is how the squirrel looks to a human
In the diagram comparing an
eagle eye and a human eye, did you notice that the eagle eye had one feature
that the human eye didn't? Birds are the only animals with this unique part,
called the pecten. What's it for? No one knows for sure. One other difference
between bird eyes and human eyes is that the retina in mammals gets a supply
of blood through tiny blood vessels. This is important for the nutrition and
health of the retina, but actually makes our vision a little poorer. Birds
don't have blood vessels in their retinas, but they DO have the pecten. Here
are some theories about why birds have this unique feature:
- To keep the
retina nourished and healthy without blood vessels.
- To keep the
fluids in the vitreous body at the right pressure.
- To absorb light
to reduce the chance of reflections inside the eye, which can distort vision
- To help birds
to perceive motion
- To provide
shade from the sun
- To sense magnetism
some data that supports the first four. The last two are simple guesses without
evidence to support them. Which of these theories makes sense to you?
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.
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?
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 eye!
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?
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.
on the diagram
of the human eye so you can see the large, labeled picture, and compare
it to your eye or a classmate's. Which of the labeled parts can you actually
see on a real eye? Which layers of the eye does light pass through to
reach the retina? Why does the pupil of a real eye look so black?
parts can we see on a real eye? If you look at a classmate's eye from the
side, you might be able to see the clear cornea sticking out like a thin
bubble. (You can't see your own cornea unless you use two mirrors, and even
then it's almost impossible!) You can see the black pupil, which is really
just the hole that lets light pass through the lens. You can see the colored
iris. The lens is too clear to see at all. You can see the white sclera.
The rest you just have to imagine!
- What eye parts
does light pass through? The cornea, aqueous body, lens, and vitreous body.
does the eye look so black? Inside the sclera of the eye is a thin
layer called the choroid coat, which has special pigments that
make it look very dark. These pigments absorb extra light inside
the eye so the only light we see is what is actually on the retina,
giving us clearer vision.
muscles (the circular ones or the radiating ones) work when the light
suddenly gets brighter? Which work when the light suddenly gets dimmer?
the light suddenly gets brighter, the circular muscles contract
to close the pupil a bit. When the light suddenly gets dimmer,
the radiating muscles contract to pull the pupil more open.
Science Education Standards
- Each plant
or animal has different structures that serve different functions in growth,
- Living systems
at all levels of organization demonstrate the complementary nature of structure