Here’s a great photo of the nictitating membrane of a red-tailed hawk. According to the Cornell Lab of Ornithology, “A nictitating membrane is a thin, translucent fold of skin that sweeps sideways across the eye from front to back, moistening and cleaning the eye and protecting its surface.” Nictitating membranes are translucent and can be found in many vertebrates, including birds, but not in humans. I have observed birds using theirs during diving, when landing in trees and bushes, when going in for a kill, as well as when relaxing. Photo taken with Nikon 200-400 zoom with 1.4x teleconverter on the Nikon D4.