This male Gadwell (Anas strepera) flashes his nictating membrane, or third eyelid. Many birds have these to clean and protect the eyes. They are semi-transparent, evidently, and some diving birds (this one’s a dabbler) have slitted ones for underwater work.
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This work by Matthew Wills is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.
Backyard and Beyond 
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