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 , cones per millimeter, giving us wonderful vision. 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. Journaling Question Let's assume eagles have exactly 1,, cones per square millimeter in their central fovea, and humans have exactly , If this was the only difference between our eyes, and if the farthest we could clearly see a 3-inch mouse was 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. 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. 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!
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.
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? 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.
Eagles, however, have retinas with cones and have a much deeper fovea—a cone-rich structure in the back of the eye. The birds are trained to fly down a long tunnel where two TV screens are kept at the end. One screen has a display of striped pattern to attract the eagle towards it and when they land on that screen, they are given a treat. Eagles have the ability to see colors more vividly than humans can.
They can even see ultraviolet light and pick out more shades of one color. Their ability to even see the UV light allows them to see the bodily traces left by their prey. Human eyes are positioned at the front of the heads, giving us a binocular vision and a peripheral vision of just degrees. We can only see complete images when we use both eyes, as closing one will block a portion of vision that was visible with that eye. The use of both eyes gives us that ability to determine the depth of field and help us realize the speed of any moving object.
Unfortunately, our peripheral vision is quite poor. Eagles have eyelids that close during sleep. For blinking, they also have an inner eyelid called a nictitating membrane. Every three or four seconds, the nictitating membrane slides across the eye from front to back, wiping dirt and dust from the cornea.
Because the membrane is translucent, the eagle can see even while it is over the eye. Eagles, like all birds, have color vision. An eagle's eye is almost as large as a human's, but its sharpness is at least four times that of a person with perfect vision.
The eagle can probably identify a rabbit moving almost a mile away.
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