How’d you like to be able to see with your eyes closed? Wouldn’t it be handy? You wouldn’t get grit in your eyes during a dust storm. Your eyes would never turn red from swimming in a pool with too much chlorine. And you’d never get poked in the eye. If only we could see through our eyelids!
It turns out that birds, and many other animals, can do just that. They share a body part that we humans lack—a fully functional third eyelid. Also called a nictitating membrane or haw, this thin sheet of tissue may be transparent, translucent, or (rarely) opaque. It slides between the cornea and the outer eyelids, offering protection from anything that might damage the eye, such as grit or drying winds.
In mammals, (but not birds) the membrane also includes a gland that produces a thin mucus, offering lubrication similar to tears.