Vision: An In-Depth Look at Eagle Eyes
Part 1

This pigeon skull shows how large bird eyes really are!

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"!

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.

 How a distant dragonfly might look to an eagle The fovea of an eagle has about a MILLION cones per millimeter. How the same dragonfly might look to a person The human fovea has 200,000 cones per millimeter 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
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.

This ruler isn't bent. Light hitting it bends as it passes from air to water. This is called 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? ( >>)
• Why might the skin right in front of the eagle's eye be bare? (>>)

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? (>>)

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.

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!

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.

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

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.