Birds have evolved a structure for protecting their eyes. Like humans, they have upper and lower outer eyelids. But beneath the outer eyelids lies an extra eyelid, called the nictitating membrane. Nictitating, for all its alliterative syllables, simply means “blinking”.

However, birds rarely blink (with the exception of owls, parrots, ostriches, and a few others) and close their eyes for sleeping by raising the lower lids. Usually unseen, however, is the third eyelid (nictitating membrane), located beneath the upper and lower lids, which moves horizontally across the eyeball.

With the notable exception of owls, parrots, piegons and very few others, birds don't blink like humans. They don't close their top and bottom eyelids completely. Birds blink with the third eyelid, and moves sideways.