Terminology

What is a Nictitating Membrane?

February 6, 2014 by Corey 5 Comments

A nictitating membrane is a third eyelid that is translucent or clear. It is used as an extra level of safety for the eye and allows the creature that has it to still see while affording its eyes an additional amount of protection. Some birds, reptiles, and sharks have nictitating membranes as well as a few mammals. (Though many mammals have a vestigial nictitating membrane that does not cover the whole eye.) Unlike the first and second eyelids, which open and close vertically and meet in the middle of the eye, the nictitating membrane moves across the entire eye horizontally.

The membrane provides protection, but it also allows the eye to be moisturized without the bird losing sight. This is extremely helpful for birds of prey flying at high speed, as it prevents their eyes from drying out. The protection aspect is probably more handy for birds that search for food underwater.

Nictitating membrane closed on Great Egret

This is a Great Egret with the nictitating membrane closed.

Nictitating membrane opening on a Great Egret

Here is the same Great Egret with the nictitating membrane only halfway across the eye.

Nictitating membrane open on Great Egret

And this is the same bird with the nictitating membrane fully open.

Some species look particularly creepy when their nictitating membrane is closed.

Nictitating membrane on a Black Vulture

Black Vultures are on no one’s list of attractive birds but when they close their nictitating membrane, which helps protect their eyes as they bury their head in a carcass, they look downright demonic.

Nictitating membrane closing on a Black Vulture

This is a close-up of a Black Vulture’s eye as the membrane closes.

I don’t think that I have heard of birds without a nictitating membrane but I have not been able to find anyone willing to say that they all have one. If anyone out there knows of any species that lack a nictitating membrane please let me know in the comments!

Nictitating membrane on a Florida Scrub-Jay

No, this Florida Scrub-Jay is not possessed by the devil, though you might have a hard time convincing a Florida real-estate developer of that!

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Written by Corey
Corey is a New Yorker who lived most of his life in upstate New York but has lived in Queens since 2008. He's only been birding since 2005 but has garnered a respectable life list by birding whenever he wasn't working as a union representative or spending time with his family. He lives in Forest Hills with Daisy and Desmond Shearwater. His bird photographs have appeared on the Today Show, in Birding, Living Bird Magazine, Bird Watcher's Digest, and many other fine publications. He is also the author of the American Birding Association Field Guide to the Birds of New York.