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My eyes tear up when cycling at 15 mph, which is nothing compared to bird airspeeds.

Do birds continuously produce lots of tears and blink a lot, or do the eyes self-moisturize from the inside without need of blinking, or are their eye surfaces just dry most of the time and they don't mind?

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  • $\begingroup$ I realise that this isn't the point of your question but wearing glasses may solve your cycling problem. $\endgroup$
    – David Richerby
    Commented Mar 20, 2015 at 0:44
  • $\begingroup$ I do wear glasses. I imagine the tearing would be much worse without them. $\endgroup$
    – SuperElectric
    Commented Mar 20, 2015 at 1:48

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Birds have a body part known as the nicitating membrane otherwise known as the "third eyelid". This part has become vestigial in humans, where it remains as the plica semilunaris.

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This image of a masked lapwing clearly shows its nicitating membrane in action, where it covers the eye in a horizontal motion. This is analogous to blinking in humans, and the membrane moisturises the eye and removes debris while the bird is in flight.

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This paper explains in detail how the peregrine falcon (the world's fastest bird) clears and moisturises its eyes when diving at speeds of over 300km/h.

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    $\begingroup$ The paper you cited also mentions that most birds' eyes have, in addition to the lacrimal gland, an extra secretory gland at the base of the nictiating membrane. At least in falcons, this secretes a fluid that is more viscous and resistant to evaporation than thinner tears. The nictiating membrane then spreads this over the eye on each blink. The membrane also has microvilli lining the inside of the leading edge that scrubs away the old layer of this viscous fluid, and any tiny debris that got stuck in it since the last blink. $\endgroup$
    – SuperElectric
    Commented Mar 20, 2015 at 1:47
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    $\begingroup$ Amphibians also have this membrane to help them visualize underwater @SuperElectric $\endgroup$
    – One Face
    Commented Mar 20, 2015 at 9:08
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Birds, like amphibians, have a third eyelid (the nicitating membrane) that helps them keeps their eyes moistened and allows them to better visualize at high speeds (or, for amphibians, underwater). The glands in birds' eyes allow them to secrete a fluid that is more resistant to evaporation than tears. The membrane also acts as a 'windshield wiper,' with each blink a birds' eyes are wiped free of excess fluid and debris.

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