How Many Eyelids Do Ducks Have?

With three eyelids per eye, some people might consider ducks odd, but a third eyelid isn't odd at all; every bird has three eyelids. The third eyelid is called a nictitating membrane, based on the Latin word nictare which means "to wink." The ability to wink and to close this third eyelid is important to the vision of ducks and all birds.

The membrane is a thin fold of skin that is nearly transparent and whitish in color. Unlike a duck's upper and lower eyelid that meet in the middle and move vertically, the nictitating membrane covers the entire eye, moving across the eye from front to back horizontally. The inner surface of the membrane contains numerous brush-tipped cells that serve to lubricate the eye with with tears.

Nictitating membranes moisten, clean and protect the surface of a duck's eyes. Because the membranes are transparent, a duck can still see when its nictitating membranes are closed. The membranes keep a duck's eyes from drying out and protect them from wind and rain when they are flying. When underwater, the membranes help a duck see better when searching for food. The membranes are said to act like in-flight and underwater goggles.

• Predatory birds, such as raptors and falcons, use the membrane to protect their eyes from thicket and other heavy cover when they pursue their prey.
• Woodpeckers employ the membranes to protect their eyes from flying wood particles when they are pecking.
• Birds aren't the only animals with nictitating membranes; reptiles, fish, dogs, cats, rabbits, camels and some prosimian primates have them too.