What makes crane eyelids unlike yours?

Cranes have eyelids that move from front to back instead of up and down like our eyelids. It is actually the crane's nictitating membrane—or inner eyelid, and the crane's actual eyelid closes from the bottom to the top. You can see the crane's eye through the closed nictitating membrane, and the crane can see through it, too. Scroll down to learn more.

Photo: Eva Szyszkoski

Birds need to be able to react to things quickly—literally in the blink of an eye—while flying, and when cranes duck their heads into water to feed, the nictitating membrane can protect their eyes from particles while they grab food. Most of the time when a bird blinks, it is closing the nictitating membrane to keep the eyeball moist and clear of dust and junk. But birds occasionally blink with the outer eyelid to keep the nictitating membrane moist and clean. When birds sleep they close the outer eyelid. What might be the reason?