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Vision: An In-Depth Look at Eagle Eyes

Introduction
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"!
This pigeon skull shows how large bird eyes really are!
Pigeon skull
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!

The 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.
 

Let's compare 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:

Eagle vs. Human Vision

how a distant dragonfly might look to an eagle

how the same dragonfly might look to a person

How a distant dragonfly might look to an eagle

How the same dragonfly might look to a person

Journaling Question
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?

Tricky fishing
This ruler isn't bent. Light hitting it bends as it passes from air to water.
Pigeon skull
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.

Facing the issues
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 Face

Human Face


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 to answer)
  • Why might the skin right in front of the eagle's eye be bare? (Link to answer)


Eyelids
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 membrane?

Journaling Question
Why do you think birds have a nictitating membrane?


Tears
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 to answer.)

Eye color
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.


Eye shape
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.

Journaling Question
Why do humans need oval-shaped eyelid openings to see well? Why do birds need round-shaped ones?


Eye size
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.

Pigeon skull
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.

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? (Link to answer.)
  • 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
Eagle Iris
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.

Activity
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? (Link to answer)


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 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.

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

  • * 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.
  • Why might the skin right in front of the eagle's eye be bare?

    Answer: 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 eye!
  • 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?

    Answer: 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.
  • 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.

National Science Education Standards

  • Each plant or animal has different structures that serve different functions in growth, survival, reproduction.
  • Living systems at all levels of organization demonstrate the complementary nature of structure and function.

 

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