Photo by Mary Holland. Check out her blog Naturally Curious.
You and I have two opaque eyelids, one above the eye and one beneath. When we blink, they meet in the middle. Some birds, amphibians, reptiles, fish, and mammals have three eyelids – two similar to ours, and a third translucent or transparent eyelid, called a nictitating membrane. This membrane moves horizontally from the inside corner to the outer edge of the eye, much like a windshield wiper, when needed for protection, to clear debris, or to moisten the eye. Although this juvenile bald eagle’s nictitating membrane makes it look blind, it isn’t. Because the membrane is translucent, the bird can still see.