Vision: An In-Depth Look at Eagle Eyes
Part 2

What Lies Beneath  

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.

Click on diagram for a larger, labeled view

Click on diagram for a larger, labeled view

Eagle eye

Human eye

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.  

Try this!

  • 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? (>>)
  • 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!
Iris: 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.
  1. Close your eyes for one minute, then open them while your partner watches.
  2. 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.
  3. 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.
  4. 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? (>>)
Eagle Iris

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.

Lens: Making Accommodations
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.

Retina: 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


Try this!
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 retina.

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 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.

Click on image to get a larger view

Click on image to get a larger view

This may be how this squirrel would look to an eagle

This is how the squirrel looks to a human


The Mysterious Pecten
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

Scientists 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?

Discussion of Journaling Questions
  • 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. 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?

    Answer: 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 body.

    Why 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.
  • Which muscles (the circular ones or the radiating ones) work when the light suddenly gets brighter? Which work when the light suddenly gets dimmer?

    Answer: 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.
Vision: An In-Depth Look at Eagle Eyes - Back to Part 1 >>