From Wild Wings Inc

Screech owls have a built-in third eyelid, called a nictitating membrane, that sweeps diagonally across the eye. It acts like a clear shield, protecting the eye from debris while still allowing the owl to see. This helps them hunt and move through dense branches without losing sight of their surroundings.

All owls have these membranes, to my knowledge. They also help keep the eyes from drying out while flying.