Birds have an upper and lower eyelid to protect their eye. They also have a thin, translucent covering called a nictitating membrane that functions as a sort of third eyelid. It sweeps across horizontally from the front of the eye next to the bill towards the back. This membrane helps to clean, moisten, and protect the eye. You can see this downy woodpecker using it to shield its eyes from possible flying debris while striking the branch as it searches for food. I have also seen a bird use this membrane to cover its eyes while feeding its young or scratching its head.

This membrane also helps keep a bird’s eye safe while underwater. I have an old binder of seminars called Bird Biology by the Cornell Lab of Ornithology that says, “In certain aquatic birds such as loons, cormorants, diving ducks, and alcids [auks, murres, and puffins] the nictitating membrane has a special central, window-like area that acts like a contact lens over the cornea.” It’s like they have built-in goggles. The world of birds is fascinating!

Nature is an inexhaustible source of wonder. I look forward to seeing you out there.

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