Birds possess remarkable visual systems, crucial for their navigation, foraging, and survival in diverse environments. A common question arises when observing these creatures: do birds blink in the same manner as humans? While birds do have eyelids, their blinking mechanism involves a unique structure that enables them to maintain clear vision even during rapid movements.
How Birds Blink
Birds blink primarily using a structure known as the nictitating membrane, often referred to as a “third eyelid.” This translucent or transparent membrane sweeps horizontally across the eye, moving from the inner corner outwards. This horizontal movement differs significantly from the vertical blinking action of human eyelids. The nictitating membrane is positioned beneath the conventional upper and lower eyelids.
While birds possess upper and lower eyelids similar to humans, their primary, rapid blink often involves the nictitating membrane. In most birds, the lower eyelid moves upwards to cover the eye when closing, such as during sleep. However, some species, including owls, parrots, and pigeons, also utilize their upper eyelids for blinking, a trait less common among other birds. The rapid and often imperceptible nature of the nictitating membrane’s movement can make it difficult for human observers to detect a bird’s blink.
Why Birds Blink
The nictitating membrane serves multiple functions. One of its main purposes is to lubricate the eye’s surface, spreading tear-like fluids across the cornea. This action helps to keep the eye moist, which is particularly important for birds flying at high speeds, where wind resistance could otherwise lead to dryness. The membrane is also effective at clearing away dust, debris, and other airborne particles, much like a windshield wiper.
Beyond lubrication and cleaning, the nictitating membrane provides protection to the eye. Birds of prey, for instance, deploy this membrane to shield their eyes when grappling with struggling prey, preventing potential scratches or injuries. Woodpeckers utilize their nictitating membrane a millisecond before impact when pecking at trees, which helps to protect their eyes from splinters and reduce the risk of retinal injury from the vibrations. For aquatic birds, the transparent or translucent nature of the membrane allows them to maintain a degree of vision even when diving underwater, functioning much like a pair of built-in goggles.