A birds 3rd eyelid

[Ira Runyan]

Two photos of a Boat-tailed Grackle taken seconds apart, the one on the bottom showing the translucent third eyelid covering the eye.

All birds have three eyelids. The two outer eyelids for birds are not used for blinking. The necessary function of lubricating the eye is instead carried out by the nictitating membrane, or third eyelid. It swipes across the cornea horizontally and is transparent allowing for full time visibility. When not in use, the nictitating membrane stays in the corner of the eye. The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds when they are under water. It is suspected that some birds may fly with the third eyelid covering the cornea of the eye, which prevents it from drying out during flight.

[Tim Abbott]

Thanks Ira, I didn't know that