nictitating membrane

Birds have a built-in lens protector that flicks across their eyes.
This third eyelid is called a nictitating membrane.
It allows the bird to still see while its eyes have extra protection.
You can see the translucency as the nictitating membrane slides across the osprey’s eye.

WALK4990 05-16-16 @ 08-17-31 Pea Ridge osprey eye