Sunday, August 05, 2007

Nothing wrong with this hawk's eye! -- AND a CORRECTION

Courtesy of

Just to forestall anxious inquiries from readers who check out many hawk websites, there's nothing wrong with this bird's eye. What you see in the photo above is the nictitating membrane, a kind of third eyelid lying under the "main" eyelids. Most birds have them. It keeps the cornea moist and clean. Most birds actually blink more with their nictitating membrane and only close their main eyelids when asleep.

[Source: The Birdwatcher's Companion by Christopher Leahy.]


Shortly after I posted this, a letter arrived from hawkwatcher Robert Schmunk. It's below, followed by one of the pictures he sent. It's hard to see the membrane even when you enlarge the picture as much as possible. That's because it's transparent, soimething I hadn't quite realized. But the bottom line still is: There's nothing wrong with this hawk's eye!

Marie, In the picture you posted today in connection with the blog post about the nictitating membrane on a hawk's eye, the membrane doesn't seem to be actually visible. That big white area is the lower outside eyelid. Attached are a couple pictures I have taken of cathedral fledglings (one last summer, one this summer) which better show the membrane. In one the membrane is in the middle of closing, so you can see that it actually moves sideways rather than up and down. In fact, I didn't even know that hawks had nictitating membranes until I took that particular picture last year. rbs
-- Robert B. Schmunk