'The Blinking Third Eyelid' is something that I just 'discovered' recently. After I took the above shots of the Great Black Hawk, I noticed that there was a translucent 'lens' on the birds' eye. After doing some research I learned that it was the blinking third eyelid, much needed to protect the birds' vision.
Not only do birds have a third blinking eyelid, other animals such as amphibians and reptiles own this unique ability too. The Blinking Third Eyelid is also called the nictitating membrane. (Nictare in latin means to blink)
I was lucky to shoot a couple of pictures of this sleepy Tree frog near the cabins. Here you can see clearly that both the upper and the lower eyelids are still open, but its eyes are still protected by the third eyelid. As it is semi transparent it gives this tree frog also the ability to maintain vision while it also protects and moistens its eyes at the same time.
Once again the Great Black Hawk. Picture on the right shows that the third eyelid is moving horizontally across its eyeball. Notice the translucency which also gives the bird a 'scary possessed look'.
The Great Black Hawk is a bird of prey, who is constantly alert. While they scan their territory they don't blink at all, but use their third eyelid to clear debris and moisten their eyes. It is also a great protection when it is scratching dangerously close to its eyes with those razor sharp claws.
A close up picture of the nictitating membrane of a Tree frog. Not only does it takes care of the eyes, but it also does look interesting.