Date: 3/7/17 9:16 am
From: Jules Evens <avocetra...>
Subject: [obol] Dipper question
That white "third eyelid" is a nictitating membrane and many waterbirds-alcids, loons, etc- have it as a means to clean and protect the eye's surface. (Reptiles and sharks have them as well.) It may also serve some function in communication in some landbirds (e.g. crows and ravens). There is a thorough discussion of its function and anatomy Cornell's "Handbook of Bird Biology (3rd ed.)
Jules Evens

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