What is a nictitating membrane?

Red-tailed Hawk

Not one...not two...but three eyelids…. how crazy is that! Hawks, like many members of the animal kingdom have multiple eyelids. The upper eyelid moves down when they blink. The lower eyelid moves up when they sleep. The third eyelid is called the nictitating membrane. It closes from left to right like a sliding glass door. The nictitating membrane is a clear membrane that is used to clean, lubricate and protect the eye when catching prey.