Yes, birds do blink, a process that differs significantly from how humans blink. Their blinking mechanism involves a complex interplay of specialized eyelids. This adaptation helps birds maintain eye health and clear vision in their dynamic environments.
How Birds Blink
Birds possess three distinct eyelids, unlike humans who have two. They have an upper and lower eyelid, similar to humans, but their primary blinking action comes from a third, internal eyelid: the nictitating membrane. This membrane is often translucent and sweeps horizontally across the eye from the inner to the outer corner.
In most bird species, the nictitating membrane is the most active and rapid of the three eyelids. While the nictitating membrane is the main player, the upper eyelid moves downwards in species like parrots, owls, and pigeons. Conversely, the lower eyelid moves upwards during drowsiness or preening. These different eyelids work in concert, with the nictitating membrane often making a rapid pass even as the lower lid rises slowly during preening.
The Purpose of Bird Blinking
Blinking serves several purposes for birds. A primary function is to keep the eye surface clean by sweeping away dust, debris, and foreign particles. This action is similar to a windshield wiper, clearing the visual field.
Blinking also helps to lubricate the eye, spreading tear-like fluids across the cornea to prevent dryness. This is especially important for birds that fly at high speeds, as wind resistance can quickly dehydrate their eyes.
The nictitating membrane also offers protection against external threats or impacts, such as when attacking prey or feeding chicks. For instance, woodpeckers tighten this membrane a millisecond before their beak impacts a tree trunk, helping to prevent retinal injury from the shock.
Why Blinking Goes Unnoticed
Bird blinking often goes unnoticed due to its speed and the characteristics of their nictitating membrane. The rapid movement of the nictitating membrane can take less than a second, sometimes matching a camera shutter speed. Small birds, in particular, blink so quickly with each head movement that the human eye cannot register it.
The nictitating membrane is often transparent, allowing birds to maintain some vision even during the blink. Unlike the opaque human eyelid, this transparency means a bird does not experience a complete loss of vision when it blinks. The swift, often clear, nature of their blink means it can be challenging to observe without high-speed video.