What is the nictitating membrane?

Have you ever seen a bird do a double-blink? Here's how and why they do it.

Sharp-shinned Hawk showing nictitating membrane

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Maybe the first time you saw it, it was on an indoor domestic cat. Or, perhaps you were watching a nature documentary and saw a shark attacking its prey, and its eye appeared to turn white or gray. It is not a defective eyelid, it is a protective piece of some animal's anatomy that protects one of their most valuable assets, their eyes.


What is it? It is the nictitating membrane.



How important is a bird's eye?


A bird's vision is possibly its best survival tool. Between finding food and avoiding predators, a bird's eyes provide information for handling each survival situation. Their dependence on their eyes comes from a lowered to nonexistent ability to smell (except for a select few species). Their ability to use their hearing for survival varies from species to species, but those species adapted to nocturnal living often have better auditory abilities. That leads us back to the importance of eyes. For most species, this is the primary tool for finding food. A slight injury to even a single eye can lead to malnutrition and death for a bird. What does this have to do with the nictitating membrane? This little flap of skin is perhaps the most important piece of PPE for a bird.


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What is the nictating membrane?


The nictitating membrane is often described as the "third eyelid" found on birds and some reptiles, sharks, and some mammals. The word nictitating comes from the root word nictate which means "blink." The membrane is a thin, translucent piece of skin that wipes laterally across the eye. In some nictitating membranes, the skin is more opaque than translucent. This eyelid is often seen when a bird is escaping a predator, or a predator is attempting to capture a prey item. In aquatic birds, this eyelid is typically closed for diving. The eyelid is often engaged during regular flight for species that possess a membrane with more translucency.



What does the nictitating membrane do?


The nictitating membrane varies its use from species to species. In most birds, the membrane acts as a street sweeper, pushing dust and other particulate matter off the cornea. It also helps keep the cornea moist. In other species, it can act as a form of communication, such as crows that will use their white nictitating membrane for displays. Some aquatic birds, like loon and cormorants, actually use the nictitating membrane as a swim goggle. There is a small window-like area in the center of the membrane that allows for the birds to see while the eyelid is closed.



Examples of the nictating membrane


Dusky Grouse showing nictitating membrane
Brown Thrasher blinking with its nictitating membrane
Least Flycatcher shows its highly translucent nictitating membrane

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Watch the nictitating membrane in action!


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