A bird’s eye has three eyelids: an upper eyelid, a lower eyelid and a nictitating membrane. The upper eyelid has limited movement, thus the lower moves up to cover the eye. The nictitating membrane or the third eyelid is a translucent membrane that moves sideways, although in the owl this membrane moves diagonally. The nictitating membrane cleans the eye and keeps it moist. It also protects the eye from particles, etc. especially when the bird lunges into the undergrowth to catch prey.
In the case of the Spotted Dove (Streptopelia chinensis), the upper eyelid can be lowered half way over the eye (above left). As observed during a preening session, the closing of the eye is achieved by moving down the upper eyelid to meet the lower eyelid that moves up (above centre and right). Apparently the lower eyelid does not move up totally, but halfway.
During foraging in grassy patches where the eyes may be submerged in vegetation, the closing of the eyelids may come into play to protect the eyes (above).
Now what has happened to the nictitating membrane?