Photo of Nictitating membrane over owl eyes
Nictitating membrane over the eyes of a Young Great Horned Owl. To protect their eyes, Owls are equipped with 3 eyelids. They have a normal upper and lower eyelid, and the third eyelid is called a nictitating membrane. It is a thin layer of tissue that closes diagonally across the eye, from the inside to the outside. This cleans and protects the surface of the eye. Image © Mack Hitch.