An eagle’s eyesight is estimated to be four to eight times more powerful than the average human’s. Birds of prey, including eagles, can see four to five times farther than we can, which means they have 20/5 or 20/4 vision. And while eagles weigh an average of only about 10 pounds, their eyes are about the same size as ours, and are larger by weight than their brains.
It is believed that an eagle can spot an ant crawling on the ground from atop a 10-story building, or a rabbit two miles away. As he drops from the sky to attack his prey, the muscles of his eyes make constant adjustments to the curvature of the eyeballs, enabling him to keep his prey in sharp focus as he makes his approach.
Eagles, like all birds, also have excellent color vision. They see colors more vividly than we do, and can distinguish more shades. They also see ultraviolet light, which allows them to detect the urine trails of small prey.
They can distinguish between five different-colored squirrels, and can find even camouflaged or hidden prey on the ground below. Bald eagles can see fish in the water while they soar or glide several hundred feet above the earth. This is more difficult than you might think, because most fish are counter-shaded — darker on top/lighter on their underside — and harder to spot from above.
From a perch at the top of a tree, the eagle can dive at 125 to 200 miles per hour to catch prey with its talons.
Each eagle eyeball moves separately, and the eyeball is so large and tightly fit that it barely turns within the eye socket. Eagles swivel their heads to locate prey and other objects of interest, moving their telephoto lens (the fovea) across their entire field of view. Once he’s spotted prey, he turns his head toward it and uses his stereoscopic vision (input from both eyes simultaneously) to judge the distance he must cover and the speed at which he must approach.
Eagles’ eyelids close when they sleep. They also have an inner eyelid called the nictitating membrane that sweeps across the eye from front to back every three or four seconds, removing dirt and dust from the cornea. The membrane is translucent, so the eagle can see through it as it slides back and forth.
By Dr. Becker