Nictitating membrane (3rd eyelid) on Laughing kookaburra, Zie-Zoo, Netherlands
Here's a crop of a 3rd eyelid, called a Nictitating membrane. I think most people have seen it one way or another on birds or reptiles, as it comes across as a bit creepy. This third eyelid opens and closes horizontally. Different from normal vertical eyelids is that they still offer *some* vision whilst being closed.
The purpose is to moisturize the eye, but also to protect it. Some birds close it so that their offspring don't pick out their eyes. Vultures may close it when going heads-first into a carcass, raptors may close them during an attack, and crocs may close them to protect against water.
The laughing kookaburra is a bird in the kingfisher subfamily Halcyoninae. It is a large robust kingfisher with a whitish head and a brown eye-stripe. The upperparts are mostly dark brown but there is a mottled light-blue patch on the wing coverts.
