Do Birds Have a Third Eyelid?
Birds have a special “third eyelid” in the corner of each eye close to the beak. This extra eyelid is called the nictitating (NICK-tih-tate-ing) membrane.
The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility.
When a bird blinks, this thin sheet of skin slides across the eyeball, wiping it clean. The membranes also act as goggles to protect the bird’s eyes when if flies fast or dives into water, and during fights with prey.
The membranes are clear, so the bird can still see where it is going. Birds also have top and bottom eyelids that can close up to protect their eyes and prevent them from drying out.
But most birds close their regular eyelids only when they go to sleep. Birds can actively control their nictitating membrane. For example, woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree to prevent shaking-induced retinal injury.