How do birds see at great speed?
In addition to the upper and lower eyelids, birds have a third eyelid. This transparent eyelid called the nictitating membrane, sits in the corner of the eye and sweeps backward. The purpose of this membrane is to clean and moisten the eye while still allowing the bird to see.
Imagine a bird without this “windshield” flying on a dusty, windy day. Closed eyelids would cause momentary loss of vision, this would not be good when plummeting earthward for a rodent or darting in and out around branches. Imagine falcons diving at great speeds and not having evolved this third eyelid; their eyes would immediately dry out. This third eyelid allows protection and vision at the same time. As for ducks, they just put on their “goggles” in order to have clear, undistorted vision while swimming underwater. We have a Master Designer who has concerns even for a bird’s eyes.
(Source: Inspired Evidence – Von Vett & Malone )