nictitating

The eagle has what is really a third eyelid, a thin translucent membrane, which naturalists call the nictitating, or winking, membrane.

Journeys Through Bookland, Vol. 4 Charles Herbert Sylvester

In addition to these it is defended by a third eyelid, called the nictitating eyelid, which is similar to that one which may be seen (if watched for) so frequently and rapidly to cross the eye of birds, e.g. of a hawk.

The Common Frog 1874

For example, throughout the vertebrated series, from fish to mammals, there occurs in the inner corner of the eye a semi-transparent eye-lid, which is called the nictitating membrane.

Darwin, and After Darwin (Vol. 1 and 3, of 3) An Exposition of the Darwinian Theory and a Discussion of Post-Darwinian Questions George John Romanes 1871

The third eyelid is left over from what's known as a "nictitating membrane," which is still present in animals like chickens, lizards and sharks.

FOXNews.com 2011

A clear, third eyelid, called a nictitating membrane, covers the otter's eyes when swimming underwater.

CITIZEN-TIMES.com - News 2008

When the eye of a bird closes with its two lids, the first to close is the nictitating membrane which closes from the lacrymal duct over to the outer corner of the eye; and the outer lid closes from below upwards, and these two intersecting motions begin first from the lacrymatory duct, because we have already seen that in front and below birds are protected and use only the upper portion of the eye from fear of birds of prey which come down from above and behind; and they uncover first the membrane from the outer corner, because if the enemy comes from behind, they have the power of escaping to the front; and again the muscle called the nictitating membrane is transparent, because, if the eye had not such a screen, they could not keep it open against the wind which strikes against the eye in the rush of their rapid flight.

The Notebooks of Leonardo Da Vinci — Complete 1452-1519 Leonardo da Vinci 1485

When the eye of a bird closes with its two lids, the first to close is the nictitating membrane which closes from the lacrymal duct over to the outer corner of the eye; and the outer lid closes from below upwards, and these two intersecting motions begin first from the lacrymatory duct, because we have already seen that in front and below birds are protected and use only the upper portion of the eye from fear of birds of prey which come down from above and behind; and they uncover first the membrane from the outer corner, because if the enemy comes from behind, they have the power of escaping to the front; and again the muscle called the nictitating membrane is transparent, because, if the eye had not such a screen, they could not keep it open against the wind which strikes against the eye in the rush of their rapid flight.

The Notebooks of Leonardo Da Vinci — Volume 2 1452-1519 Leonardo da Vinci 1485

a small crescent-shaped fold of skin; this is the last relic of a third inner eye-lid, called the nictitating (winking) membrane.

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

This nictitating membrane protects the bird from injury while going down into long grasses to grab prey.

Rich Wolf: Boulder Predator Spotted in New York City 2010

Their eyes are so important to birds that, like various reptiles, sharks, and amphibians, birds have a transparent or translucent third eyelid, the nictitating membrane, to protect and moisten the eyes while retaining visibility.

Birdology Sy Montgomery 2010