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 carnivorous bird in the kingfisher family Halcyonidae. Native to eastern Australia, it has also been introduced to parts of New Zealand, Tasmania and Western Australia. Male and female adults are similar in plumage, which is predominantly brown and white. A common and familiar bird, this species of kookaburra is well known for its laughing call.