For birds to survive, they need keen eyesight, to find food, to navigate their environment, find a mate and avoiding becoming something else’s meal. So it is critical to protect their eyes, especially when flying through the air and through vegetation where there is an increased risk of something hitting their eyes and causing abrasions. In addition, birds of prey have to worry about their prey injuring their eyes after they catch it.
One adaptation that some birds have that helps with this problem is my having a third eyelid, also called the nictitating membrane. This feature can be seen in other groups of animals besides birds also, including reptiles, sharks, and mammals such as cats, camels and seals.
So what exactly is this nictitating membrane? It is a translucent eyelid that moves from side to side, unlike what you would see from your eyelids that open on the top and bottom. The main purpose of these third eyelids is to protect and moisten the eyes. Birds are able to fully control this membrane and depending on the species, its primary use may vary. A kingfisher may use their nictitating membrane as goggles when diving into the water to catch a fish, or a woodpecker may use theirs to protect their eyes and prevent impact injuries to their eyes as they pound at decaying wood when looking for food or excavating a nesting cavity.