Nictitating Membrane

The most important sense that birds possess is vision. Their extraordinary eyesight is two to three times greater than ours and in the case case of raptors and owls even greater than that. In order to protect their eyes from dust particles and injury they have 3 eyelids, an upper, a lower, and a thin transparent eyelid called a nictitating membrane. Nictitating membranes have their own lubricating duct, similar to our tear duct, used to clean the surface of the eye and protect it. Raptors use their nictitating membrane for protection from their own sharp talons, such as when they are scratching their heads, as well as when they are feeding their chicks. Woodpeckers use theirs when they are drilling to protect their eye from flying wood chips as well as to help the eye stay in its socket!

In most birds the nictitating membrane is translucent, which allows light to pass through but does not allow them to see clearly, which is not a big deal as it is only for split second. In diving ducks, cormorants, grebes, pelicans and loons, i.e., birds that fish underwater, it is a transparent, enabling them to see under water.

I photographed this Gray Catbird yesterday as it was bathing, and the nictitating membrane is clearly visible.

This image has caught the nictitating membrane in mid-blink as it moves horizontally across the eyeball.

The translucent nictitating membrane fully closed.

In the blink of an eye it is gone!