From Werling Wildlife
WHAT'S UP WITH THOSE EYES!?!? It's a nictitating membrane!
The nictitating membrane, also known as the third eyelid, is a transparent or translucent membrane found in some animals, particularly birds, reptiles, and some mammals. It acts as an additional eyelid, providing protection and moisture to the eye while still allowing for some vision. In birds of prey like this eastern screech owl, the primary purpose is to shield the eye from dust and debris. It's basically their version of windshield wipers!
Humans have a vestigial structure called the plica semilunaris, which is a small, pink, crescent-shaped fold of tissue in the inner corner of the eye. This structure is a remnant of a once functional nictitating membrane, but it no longer serves the same protective or lubricating purpose as in other animals.
