Birds’ eyes can change size for a few different reasons. The most common reasons are to help them see better in different lighting conditions, express emotions, and clean their eyes. Birds have a structure called the iris which controls how much light gets into their eye. When the iris expands, the pupil (the black center part) gets bigger, letting in more light. When the iris contracts, the pupil gets smaller to let in less light. Being able to change the pupil size allows birds to see well in both bright and dim conditions. Some bird species also use their eyes to communicate or express emotions. An expanded pupil may signal fear or interest. Finally, birds have a third eyelid called a nictitating membrane that they can sweep across the eye to clean and lubricate it. When birds do this, the eye may appear to shrink or get smaller temporarily. Let’s explore these reasons in more detail.
Seeing in Different Light Conditions
Birds’ ability to adjust their pupils helps them see clearly in varied lighting conditions. In bright daylight, birds will constrict their pupils to tiny slits to avoid being overwhelmed by too much light. At night or on cloudy days, they can open their pupils wide to let in more light and see as clearly as possible. Most bird species have pupils that can dilate and constrict over a wide range, allowing their eyes to go from big to small very quickly. Being able to go from a wide, open pupil to a narrow slit allows birds to transition smoothly from dim to bright light. This protects their vision and lets them see the finest details at all times of day.
Some birds that are active at night, like owls, have huge pupils that can open even wider than other species. Their eyes look exceptionally big in the dark because their pupils dilate to let in as much light as possible. Other birds that live in darker environments may also have proportionally larger eyes to gather more light. So a key reason birds’ pupils and eyes change size is to adapt to the ambient lighting and see the world around them as clearly as possible.
Expressing Emotions
In some bird species, a change in pupil size may be associated with certain emotional states. When birds feel threatened, surprised, or excited, their pupils often dilate and their eyes open wider. This helps them take in more visual information and be alert to dangers or new stimuli. Conversely, relaxed, comfortable birds often have their pupils constricted to small slits. So pupil dilation can signal higher arousal or vigilance.
Courtship displays are another context where birds may consciously or unconsciously alter their pupil size. A male bird pursuing a female may dilate his pupils to signal his interest and attention. The receptive female may reciprocate with dilated pupils of her own. These cues amplify each bird’s body language and help coordinate breeding behavior. So pupil dilation and eye expansion can indicate moods and facilitate social interaction in some species. It provides visual feedback on how birds are reacting to different situations.
Cleaning the Eyes
Birds have a third eyelid called the nictitating membrane that can sweep across the surface of the eye to clean or lubricate it. When the nictitating membrane closes, it covers the eye like a shutter, making the eye appear to shrink temporarily. Birds may close this third eyelid frequently while flying to protect their eyes from wind and debris. The motion of the membrane spreads tears and other secretions over the surface of the eye, keeping it moist and healthy. So periodically, a bird’s eyes may seem to get small as the nictitating membranes close for cleaning and maintenance. However, this is just a temporary effect that passes as the third eyelids reopen.
Differences Between Bird Species
While adjustable pupils are the norm in birds, there are some differences between various species. Here are a few examples:
Owls
Owls have some of the largest pupils relative to their eye size of any bird. Their pupils can open very wide, nearly filling the whole eye, to maximize light capture at night.
Hawks and Eagles
Diurnal raptors like hawks and eagles have slit-shaped pupils that can vary from very narrow to fairly wide. This helps them scan the ground for prey while filtering bright daylight.
Woodpeckers
Woodpeckers have rectangular-shaped pupils that work like slits to cut glare while peering at tree bark. Their pupils allow binocular vision from a side angle.
Seabirds
Seabirds that forage on water often have round, moderately-sized pupils. These work well across a range of light conditions at sea.
Songbirds
Songbirds tend to have round pupils that can open fairly wide to see well in forests and shrubbery. Their pupils constrict to small points in bright light.
So while all bird species can adjust their pupils, the shape and flexibility varies based on ecological niche and lifestyle. This tuning helps each species see optimally in their natural environment.
The Iris Muscles
So how do birds alter the size of their pupils so dramatically? The secret lies in the iris – the colored part of the eye surrounding the pupil. The iris is made of two sets of muscles:
Sphincter Muscles
– These are circular muscles that wrap around the pupil and can constrict to make the pupil smaller
Dilator Muscles
– These radiate out from the pupil and can contract to make it larger
By flexing these opposing muscle groups, birds can rapidly adjust the amount of light entering their eyes. The sphincter muscles squeeze the pupil down to a pinpoint in bright conditions. When the dilator muscles contract, they overwhelm the sphincters and cause the pupil to expand. It is this dynamic interplay between the two sets of muscles that allows a bird’s pupil to go from tiny to huge in just seconds!
The iris muscles are controlled by reflex circuits in the birds’ brains that respond to the ambient light. When light levels increase, the pupil constriction reflex kicks in to prevent overload. Dim conditions trigger pupil dilation. This happens unconsciously to optimize vision. But birds may also learn to control the muscles voluntarily to signal emotional states like interest or alarm. Either way, this muscular iris provides the physical apparatus for birds to alter their pupil size.
Advantages for Birds
Having dynamic pupils and the ability to go from big to small eyes confers many advantages for birds:
Better Vision
Adjustable pupils allow birds to see well in all light conditions and scrutinize their environment.
Communication
Subtle pupil changes provide visual cues on mood and intent between birds.
Courtship
Dilated pupils help display interest and facilitate pair bonding.
Alertness
Enlarged pupils indicate heightened arousal and vigilance.
Cleaning
Closing the nictitating membrane cleans and lubricates the eyes.
So pupil flexibility is not just a novelty – it serves many important purposes for avian vision, social interaction, and health. The ability to go from big to small improves birds’ sight, communication, courtship, and overall wellbeing.
Pupil Shapes
While size changes are key, pupil shape also varies in birds:
Slitted
Common in raptors like hawks and eagles. Minimizes glare while allowing binocular vision.
Round
Typical of owls, seabirds, and songbirds. Works well in many conditions.
Rectangular
Specialized for woodpeckers to focus on bark while hammering.
So pupil form can complement size adjustments and be adapted to different lifestyles. But the overall ability to dilate and contract the pupil is universal across birds.
Conclusion
In summary, birds’ eyes can rapidly go from big pupils to small pinpoints thanks to the adjustable iris. This helps them see in diverse lighting, communicate visually, find mates, stay vigilant, and keep their eyes clean and healthy. Pupil size changes are accomplished by sets of muscles that alter the aperture of light entering the eye. While all birds share this general ability, the shape and flexibility of the pupil can vary by species and lifestyle. So next time you see a bird’s eyes dilate or contract, know that this remarkable mechanism allows them to thrive across environments and situations.