Birds have a “third eyelid” – a nictitating membrane which is located between the cornea (the membrane covering the eye surface) and the upper and lower eyelids. This thin, moveable membrane is used for protecting, lubricating, and cleaning the eye. The upper eyelid moves when a bird blinks. The lower eyelid closes when the bird is sleeping. The nictitating membrane opens and closes from left to right, while the upper and lower eyelids move up and down to open and close.
While working on this portrait (the bird seemed to have had enough of me right about here) I captured the exact moment the hawk blinked. With his nictitating lid.
I usually don’t use photographs of birds in this state. Here the timing made it perfect to see the difference.
This is why I love working with animals. They can’t complain about how they look in a shot !