Taking a Close Look
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What was your first reaction to this bird? Did you think he might be blind? His eye lacks the normal bright golden color, and the iris is not visible.

What's happening here?
This eagle has his nictitating membrane closed over his eye.

Did you know that eagles (and other birds) have 3 eyelids! The outside two are the ones we usually see. On eagles, the bottom eyelid is bigger than the top eyelid, so they blink up instead of down. The third eyelid, an inner eyelid, is called the nictitating membrane. This eyelid is transparent, and sweeps across the eye from side to side. They evolved this extra layer of protection to keep their eyes moist and clean while still being able to see.

Eagle eyes have a constant flow of tears (just like human eyes) that keep them clean and moist. The nictitating membrane sweeps from the inside corner of they eye to the outside edge carrying a film of moisture all across the eye.

Usually, this membrane stays in the corner of the eye when not in use. Humans' nictitating membrane is permanently folded into that corner; it's the visible pink nub.Can you see it in your classmate's eyes?

In these photographs the bird may have been caught preening, or cleaning his talons, and closed the nictitating membrane to protect his eyes from that dangerous set of claws!
Photos: Jon McRay