An In-Depth Look at Eagle Eyes
In 1578, a writer named Guillaume de Salluste, Siegneur Du Bartas
described eyes as, "these lovely lamps, these windows of the
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.
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.
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? (>>)
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
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.
your eyes for one minute, then open them while your partner watches.
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.
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.
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.
muscles (the circular ones or the radiating ones) work when the
light suddenly gets brighter? Which work when the light suddenly
gets dimmer? (>>)
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."
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
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.
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!
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.
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 retina.
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 before,
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:
keep the retina nourished and healthy without blood vessels.
keep the fluids in the vitreous body at the right pressure.
absorb light to reduce the chance of reflections inside the eye,
which can distort vision
help birds to perceive motion
provide shade from the sun
have 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 In-Depth Look at Eagle Eyes - Back
to Part 1
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
What 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
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?
When 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.