The Eye Of An Eagle

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! 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. 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.