(cont…) One question that is frequently asked by kids at a Liberty presentation is, “Do birds blink their eyes?” The answer at first seems simple: YES! But beyond that, birds not only have a top and bottom eyelid like we do, but they have a third eyelid as well. I recently learned that humans (and primates in general) are in the minority of species that do not have this feature, called a “nictitating membrane” from the Latin word nictare, to blink. Actually, we DO have a vestigial version of this called the plica semilunaris, but it serves a different purpose in our eyes. Birds, reptiles, fish and a few mammals have a full nictitating membrane that is functional for several things (in polar bears, it prevents snow blindness!).
In most birds, it is usually translucent or transparent, but in the case of owls, it is opaque. It provides moisture to the eyeball plus protection from physical damage from foreign objects as it is drawn horizontally across the eye from the beak to the ear. A peregrine falcon uses this membrane to protect its all-important eyes from damage as they stoop at 200mph after their prey. Raptors of all types will close this membrane just before they strike their prey in case the target animal tries to defend itself while being attacked. Mother birds will close this eyelid as they feed their nestlings to prevent an over-eager chick from scratching her eye while she places food into their gaping beaks.
Some birds, like ospreys and some diving ducks, loons, and auks, use their nictitating membranes for water goggles as they submerge themselves after their prey. Some actually have a clear lens-shaped window with a high refractive index that amounts to a “contact lens” when they are under water.
The true eyelids of birds are used for sleeping in most avian species. Owls use their upper lids to blink, and do so independently right and left (a fact that seems to amuse many young Liberty program attendees!). They use their lower eyelids mostly for sleeping. Of all animal species, including man, birds alone close their eyes when they die.