Birds must maintain the health and clarity of their eyes, requiring regular cleansing and moistening. While the common understanding of a blink involves the vertical movement of upper and lower eyelids, avian blinking operates on a fundamentally different, three-part system. Birds blink constantly, but their primary method is distinct from the vertical lid closure seen in humans. This specialized system allows them to keep their visual field clear and protected while navigating complex, high-speed environments.
The Basic Avian Eyelids
Birds possess a pair of upper and lower eyelids similar to those found in mammals. These structures are generally less prominent and mobile during a bird’s waking hours compared to human eyelids. For the majority of avian species, the lower eyelid is responsible for most of the vertical movement when the eye closes.
The primary function of these outer eyelids is to protect the eye during periods of rest and sleep. When a bird is drowsy or asleep, the lower lid typically rises to meet the upper lid, completely covering the eyeball. Some exceptions exist, such as owls and parrots, which exhibit more movement in their upper eyelid. These conventional eyelids also provide structural protection from injury.
The Specialized Nictitating Membrane
The most unique and frequently used component of the avian blinking apparatus is the nictitating membrane, often referred to as the third eyelid. This structure is a thin, translucent tissue located beneath the outer eyelids, starting at the inner corner of the eye. Its movement is entirely different from the vertical closure of the outer lids, as it sweeps horizontally or diagonally across the cornea.
The nictitating membrane has specialized, active muscular control, allowing the bird to draw the membrane across the eye rapidly and at will. Its main roles are to lubricate the cornea, sweep away debris and dust, and distribute the precorneal tear film across the surface.
Because the membrane is translucent, a bird can still maintain a degree of vision even while the third eyelid is drawn across the eye. This ability is especially important for aquatic birds, like cormorants, which use the membrane like a built-in goggle for underwater vision. This quick, near-transparent sweep facilitates cleansing without significantly interrupting the continuous visual input necessary for survival.
Blinking Speed and Visual Performance
The rapid movement of the nictitating membrane is directly linked to a bird’s need for high visual acuity, especially during flight. These quick, phasic blinks are necessary to refresh the tear film without obscuring vision for more than a fraction of a second. Studies on flying birds, like grackles, have shown that blinks are significantly quicker while airborne than when the birds are on the ground.
This rapid blinking protects the eye from environmental hazards at high speeds. Peregrine falcons, for example, repeatedly deploy the nictitating membrane to clear debris and maintain moisture during high-velocity dives, which can reach speeds exceeding 200 miles per hour. The strategic use of the membrane is also evident in ground-foraging birds, such as wood pigeons, which close the third eyelid with each peck to shield the eye from potential injury.
Furthermore, research indicates that birds strategically minimize blinking during periods requiring intense visual focus, such as the moment just before landing. The speed of the nictitating membrane allows for this strategic blinking pattern, ensuring that the necessary cleaning and moisturizing functions are performed with minimal impact on the bird’s visual tracking. This balance between eye health and visual performance underscores the membrane’s importance in the avian lifestyle.