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Mammals have an upper lid and small mass of tissue in the upper corner known as nictitating membrane. This helps keep the eyes clean; birds can moisten their eyes in flight using this membrane without blinking. How does this work?

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Because the nictitating membrane secretes a fluid that is resistant to evaporation. Therefore, birds need not blink their eyes during flight.

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    $\begingroup$ Please do add some solid references. $\endgroup$ – L.B. Oct 28 '15 at 20:09

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