May 04 2009
Question: Why are her eyes white?
Question: I was watching the web camera and saw the adult peregrine’s eyes go white. What does this mean?
Answer: When a peregrine’s eyes look white it’s because he has closed the nictitating membrane – his third eyelid – to protect and moisten his eyes. Because it’s translucent he can still see.
The third eyelid is especially important when peregrine falcons dive on prey. In a fast dive they can travel 200 miles per hour so they blink the nictitating membrane rapidly to keep their eyes moist and clean. They can’t afford to get grit in their eyes at that speed.
Many birds and mammals have nictitating membranes including owls and cats. Humans don’t have this eyelid any more, just a vestigial bit in the corner of our eyes closest to our noses.
Here, Dorothy has closed her third eyelid and is probably taking a nap as she stands over her chicks. She can still be watchful, even with her eyes “closed.”
(photo from the National Aviary webcam at the University of Pittsburgh)
