Q: I’ve heard that hawks have an extra eyelid or membrane that protects their eyes while they hunt. Is that true? Do other animals have extra eyelids? [Mark, San Jose]
A: According to the Cornell Lab of Ornithology’s authoritative Handbook of Bird Biology, “in raptors and other predatory birds, the nictitating membrane protects the eyes as the bird pursues prey through heavy cover, such as a blackberry thicket.” But that’s only one of the many functions of this “second eyelid.”
In fact, all birds have one: a transparent fold of skin that sweeps sideways across the eye to clean and moisten the cornea. The membrane’s inner surface is covered with brush-tipped cells that lubricate the cornea’s surface with tears. Some birds, including falcons, have a special gland that produces a thick fluid to moisten the cornea. When a peregrine falcon stoops on its prey, flying at up to 240 miles per hour, the nictitating membrane keeps its eyes from drying out. It can be deployed as a bird flies against the wind, or in rain or snow. It may also protect a raptor’s eyes from the sharp claws or teeth of flailing prey and the sharp beaks of its own nestlings.
Diving birds–loons, auks, diving ducks–close their nictitating membranes when they plunge. A clear lens-shaped window in the center of the membrane may help the birds see better underwater. The nictitating membrane of the American dipper, an amphibious songbird common along Sierran streams, has been described as cloudy, milky, or opaque–an odd feature for a bird that preys on creek-bottom insects. Field studies on this seem to be lacking.
Local raptor expert Hans Peeters says the nictitating membrane of owls is semitransparent. Whatever the owls use this for, it’s not protection against bright light: According to one report, a captive barn owl didn’t close its membranes when facing the sun or staring at a glowing lightbulb.
In other variations, the membrane of black-billed and yellow-billed magpies has an orange spot that males flash at each other during dominance contests. Woodpeckers close their unusually thick nictitating membranes just before beak hits wood. UC Davis ophthalmologist Ivan Schwab says that action helps cushion their eyes against the impact and guard them from flying chips.
This is clearly a handy piece of skin. Reptiles also have nictitating membranes, as do some groups of mammals, including rabbits, carnivores, and some prosimian primates. But the membrane in great apes and humans, called the plica semilunaris, is vestigial: immobile and barely visible in the corner of the eye.
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