If I had eyelids like a bird, I wouldn’t have to worry about the Wyoming wind. Raptors and other birds not only have two eyelids, they have a third eyelid as well. Two of their eyelids are like ours, moving up and down to close their eyes. Like us, owls close their eyes by moving the top eyelid down, while other birds move the bottom up. In most avian species, the true eyelids are used for sleeping. In owls, however, the upper eyelid also closes when the owl blinks.
The third eyelid is called the nictitating membrane. In addition to birds, nictitating membranes can also be found in some reptiles, mammals, and sharks. It is this membrane, or third eyelid, that birds primarily use to blink. In most birds this is a thin, semi-transparent eyelid which moves horizontally or diagonally from the inside to the outside of a bird’s eye. In owls, however, the membrane is opaque.*
A quick blink with this third eyelid cleans and moisturizes the eye. When closed, the membrane also helps to protect the bird’s eye while, in most species, still allowing some vision depending on the amount of transparency of the nictitating membrane. Many birds use this membrane to protect their eyes while feeding their chicks and raptors may close them when attacking prey. Some birds use the membrane like we use a pair of goggles. Peregrine falcons are famous for their fast dives and are known to use the nictitating membrane to protect their eyes from wind and dust as they rocket toward their prey. Loons close the membrane when diving into the water after fish. In vultures, the membrane will protect the birds when putting their heads inside the carcass they are feeding on. Without the membrane, forest raptors could more easily scratch their eyes while chasing prey. Red-tailed hawks hunting from a high, open perch may use the nictitating membrane to protect and clean their eyes during high winds, rain, and snow. So if I had eyelids like a bird, I wouldn’t have to worry about the dust stirred up by our Wyoming wind.
This video will give you a close up view of a juvenile eagle closing its lower eyelid, as well as the movement of the blue colored nictitating membrane (the 3rd eyelid).
https://www.youtube.com/watch?v=7oAN7L2_E6U
*Reference for opaque membrane in Owls:
Delaware Valley Raptor Center
And The Barn Owl by D.S. Bunn, A.B. Warburton, and R,D,S, Wilson, page 35.
Bald Eagle Photo by: Pat Gaines, Attribution License http://tinyurl.com/pfgm3sc
Ostrich photo: By Len Radin, Attribution License https://www.flickr.com/photos/drurydrama/11957058896/in/photolist-jdB42S-2rpdQy-qAvbcX
All other photos are my own.
