Birds do not blink in the same way that humans do using their upper and lower eyelids. Instead the eye is kept clean and lubricated by a third, usually concealed eyelid. called the nictitating membrane. Fully developed nictitating membranes are also found in fish, reptiles, amphibians and mammals including occasionally in primates such as lemurs.
The nictitating membrane gets its name from the Latin word “nictare” which means “to blink” but instead of moving up and down, it sweeps across the eye horizontally like a windscreen wiper and is usually translucent.
When birds are moving their heads they will draw the nictitating membrane more frequently across their eyes than when they are still, possibly to block out the blurred images that result from rapid head movements. Small birds can blink so fast with each head movement that human eyesight is unable to register it.
Birds are able to control their nictitating membranes and for some time it was thought that they could be used as contact lenses when aquatic birds were underwater. However, after numerous studies and careful measurements it has been found that the nictitating membrane does not alter the reflective state of the eye and its curvature is virtually the same as the cornea.
Some diving birds such as cormorants have small transparent windows in their nictitating membrane so that they can see when they are swimming underwater.
The nictitating membrane can also be used to protect the eye from injury during visually guided behaviour. For example wood pigeons will often close their nictitating membranes during each peck as they forage near the ground.
Adult birds of prey will cover their eyes with the nictitating membrane to protect them when they are feeding their chicks and when peregrine falcons go into their 320 km/h dives they will repeatedly blink with their nictitating membrane to clear debris from their eyes and keep them moisturized.
