Do birds eyes water when they fly?
As it turns out, they don’t. Through an adaptation, their vision is unimpaired during flight. Here’s how it works. Birds have a nictitating membrane, the so-called inner eyelid that dogs, cats, and Vulcans show when they sleep. Its function is to protect and clean the external surface of the eye. It sweeps transversely across the eye from the nasal passage area towards the ear. As it does, it releases lacrimal fluidtearson to the surface of the eye through a duct located between the nictitating membrane and the surface of the eye. In this manner, the lacrimal fluid lubricates and protects the surface of the eye by maintaining a constant thin layer of fluid between the two surfaces where friction would occur without wasting excess fluid. Additionally, the leading edge of the nictitating membrane as it sweeps across the cornea acts as a squeegee polishing the cornea, limiting the amount of lacrimal fluid left on the eye, and sweeps foreign objects off the eye before it. Mammals possess e
