Thinner and more transparent than the fleshier upper and lower eyelids we’re all familiar with, the nictitating membrane allows some degree of visibility while simultaneously protecting the eye. The bills (and therefore the heads) of birds are vulnerable during foraging, flight, excavating, and diving, while feeding young, and gathering material for nests, etc., so extra eye protection is quite valuable.

The nictitating membrane is widespread in nearly all the vertebrates, including fish, amphibians, reptiles (which include birds), and mammals. In humans and other primates, they exist only as a vestigial structure.

https://www.birdnote.org/explore/field-notes/2020/03/natures-goggles-nictitating-membranes