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Nature Notebook – Nictitating Membrane

While recently transferring an injured adult barred owl to a wildlife rehabilitator, I was fascinated by its’ eyes. Owls, among many other animal species, possess a third eyelid, called the nictitating membrane. This transparent membrane, present in most birds, protects their eyes from dust, dirt, and debris. Birds can voluntarily control this horizontal-moving lens and often use it while flying, hunting, and feeding their young.

Peregrine falcons use this membrane to keep their eyes moistened and free of debris when flying almost 200 miles per hour after prey. Woodpeckers tauten the membrane just before hammering into a tree to protect their eyes from too much jolting. The nictitating membrane is also helpful for animals during inclement weather, protecting their eyes from snow, rain and wind.

This membrane is not only present in birds. Many amphibians, reptiles, fish and some mammals possess this trait. It is especially helpful for diving animals, allowing them to see underwater while protecting their eye. Even our dogs and cats possess this membrane, though it’s not always visible. Humans still have the vestigial remnant of this membrane, called the plica semilunaris, but is now only a small fold of tissue in the inside corner of the eye.