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.
eagle has his nictitating membrane closed
over his eye.
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.
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.
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?
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!
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