How’d you like to be able to see with your eyes closed? Wouldn’t it be handy? You wouldn’t get grit in your eyes during a dust storm. Your eyes would never turn red from swimming in a pool with too much chlorine. And you’d never get poked in the eye. If only we could see through our eyelids!
It turns out that birds, and many other animals, can do just that. They share a body part that we humans lack—a fully functional third eyelid. Also called a nictitating membrane or haw, this thin sheet of tissue may be transparent, translucent, or (rarely) opaque. It slides between the cornea and the outer eyelids, offering protection from anything that might damage the eye, such as grit or drying winds.
In mammals, (but not birds) the membrane also includes a gland that produces a thin mucus, offering lubrication similar to tears.
When we blink, our eyelids meet together from above and below our eye. The nictitating membrane, on the other hand, closes from the side, much like a sliding patio door. And like a glass door, there’s even a cartilage “frame” that holds the membrane rigid, keeping it from wrinkling or folding in on itself.
When you consider the list of species with this extra feature—it includes fish, amphibians, reptiles, birds, and many mammals—we find that humans are in the minority. Polar bears and seals have one; it protects their eyes from subfreezing temperatures and UV radiation, helping to prevent snow blindness. Camels have one too; think how helpful it would be during a desert sandstorm. Aquatic animals (ducks, manatees, and beavers, for example) close their nictitating membrane while underwater. In this alligator at right, the membrane reflects blue.
Interestingly, a nictitating membrane is rare in primates (it’s found only in lemurs and other “primitive” [lorisoid] primates), although there are vestigial parts in the eye structure of many mammals, including us. (Rarely, a human is born with a more developed membrane. It interferes with vision and requires surgical removal.)
For some mammals (such as cats and dogs), movement of their third eyelid is involuntary, often closing when the animals are relaxed. Birds, on the other hand, can control their membrane, and they make good use of it. That protection is essential. Highly dependent on their eyesight, a visually impaired bird won’t last long.
Flying birds move at high speeds, often through dense foliage or thick forests. They can’t afford to close their eyes, even for the fraction of a second it takes to blink. That’s why their nictitating membranes are typically transparent or semi-transparent. 
Imagine a peregrine falcon plummeting at 200 miles per hour, and how drying the air rushing past their eyes would be—yet, they need to stay focused on their prey. Perhaps that prey is hiding in a bush—full of small twigs that could easily scratch a cornea. And not all prey passively accepts being eaten. There are sharp teeth and claws on both sides of the equation.
Even feeding your chicks is risky. Those little nestlings are anxious for their dinners, and they have sharp beaks. The danger of being pecked in the eye is highlighted by the fact that the parents keep their membranes mostly closed during mealtimes.
Woodpeckers exert phenomenal pressure on their eyes while pecking. Studies have revealed that, to protect their retina, they close their nictitating membrane a fraction of a second before they hit the tree (or telephone pole or chimney flashing). It’s similar to the way we’d tighten our seat belt if we see a crash coming.
I admit to feeling a bit cheated when it comes to protecting my eyes. I also go out in dust storms and freezing weather. I love to swim, and often open my eyes underwater. And I find myself squinting when the sun comes out after a big snowstorm. Sure, we can use goggles, dive masks, and sunglasses, but wouldn’t a third eyelid would come in handy?
 According to And The Barn Owl by D.S. Bunn, A.B. Warburton, and R.D.S. Wilson, the nictitating membranes of owls are opaque.
 Wygnanski-Jaffe T, Murphy CJ, Smith C, Kubai M, Christopherson P, Ethier CR, Levin AV. (2007) Protective ocular mechanisms in woodpeckers Eye 21, 83–89.