Nictitating Membranes
If you look closely, you’ll see that part of this American Crow’s eye appears white. This white (actually transparent) part of the eye is referred to as a “third eyelid,” or nictitating membrane. Many animals, including some fish, mammals, reptiles, amphibians and birds, have nictitating membranes. Instead of moving up or down like the upper and lower eyelids, they move horizontally across the eye, much like windshield wipers. These membranes lie under a bird’s eyelids. When not in use, they are curled up in the inner corners of a bird’s eyes, but can be quickly drawn across the eyes to protect, clean and moisten them without shutting out the light.
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