Vision: Adaptations and Capabilities
A Golden Eagle soars high across the windswept plains and hills of her fifty square-mile territory. Prey is scarce here, and finding such a ‘needle in a haystack’ would be next to impossible for us, but her eagle eyes can spot the movement of a rabbit from over a mile away.
Humans may have excellent vision, but birds deserve the prize for the best eyes in the natural world. Avian eyes aren’t all the same, of course, and each species has evolved adaptations that suit their particular lifestyle and foraging strategy.
Some nocturnal birds, for example, are more than twice as sensitive to light than we are, while diurnal birds can see light in the ultraviolet spectrum, and some species can even see the entire 360 degrees around their heads!
We may never know what it’s like to see out of any eyes except our own, but we do know that Eagles, and all other birds, perceive the world in vastly different ways from ourselves. In this guide, we’ll be diving into some of these avian superpowers and learning how birds rely on them for day-to-day survival.
A Spectrum Beyond Our Own
Ultraviolet Vision and Polarized Light Detection
Birds, and many other non-mammalian animals, are tetrachromatic, which allows them to see colors in the ultraviolet (UV) range of the spectrum, revealing a world unseen to the human eye.
Seeing in UV allows birds to show off an even wider range of bright plumage colors during courtship, but it also helps them find food. Seeds and berries, for example, show strong contrast in UV, making them much easier to detect.
Many flowers have lines and markings on them to attract pollinators and direct them to the nectar, and scientists have discovered that many flowers have distinct patterns visible only in UV. These are most important for insect pollinators like bees, but they may also guide specialized nectar feeders like Hummingbirds.
There is a lot more to uncover, but some evidence suggests that birds can detect polarized light and use it for compass calibration, which is important for migratory species that must navigate between their summer and winter home ranges. Birds may even use polarized light to detect water bodies.
A Steely-vented Hummingbird. Many flowers have lines and markings on them to attract pollinators and direct them to the nectar, and scientists have discovered that many flowers have distinct patterns visible only in UV
Precision and Acuity
High Resolution
Birds have very large eyes, with an impressive density of photoreceptors (rods and cones) in their retinas and a high ratio of receptor cells to ganglia.
The world’s largest bird may have the biggest eyes of any land animal, but the highest resolution is found in Eagles. These large birds of prey have similar-sized eyes to humans but nearly five times as many color-sensitive cone cells!
Multiple Foveae
Almost all birds (chickens are an exception) have a pit or depression in their retina called the central fovea that holds an especially high density of photoreceptors.
Agile hunting birds like Barn Swallows, House Martins, and diurnal birds of prey have a second fovea in each eye called the temporal fovea, which is used in binocular vision. These bifoveate species have multiple areas of especially acute vision and improved depth perception and binocular vision.
Dynamic Range Adaptation
Bird eyes must adjust as light intensity changes throughout the day and night due to factors like cloud cover, moon phase, and the rotation of the Earth.
These changes happen pretty predictably or at least relatively slowly, but birds move fast, so their eyes must adapt to changing light conditions quickly when flying through sunlit and shaded areas.
Birds dilate or contract their iris to vary the amount of light that reaches the retina and maintain their vision in fast-changing light levels. Unlike mammals, birds control their pupil size with striated muscles, which suggests that they may even have some voluntary control over their pupil size.
A Barn Swallow. Agile hunting birds like Barn Swallows, House Martins, and diurnal birds of prey have a second fovea in each eye called the temporal fovea, which is used in binocular vision
Depth and Motion Perception
Binocular Vision
Stereopsis, or binocular vision, occurs when the field of each eye overlaps, allowing birds to judge depth, distance, and the size of objects. All birds have some binocular vision, but this ability is especially important for birds that hunt moving targets at high speeds.
Predatory birds like raptors and Owls have eyes on the front of their face for enhanced binocular vision because there is more visual overlap. Prey birds with eyes on the side have limited binocular vision, but they can still judge depth and distance by bobbing their heads.
Motion Sensitivity
Research has shown that some small passerines detect flashing lights at up to 145Hz (flashes per second), which is over 50hz faster than humans! That means they can take in a whole lot more information per second than we can, almost as if they are watching the world in slow motion.
Such a high ‘frame rate’ allows them to detect even the briefest of movements, and fast eyes have obvious benefits for birds that hunt fast-moving or elusive prey.
However, motion sensitivity also helps prey birds detect the measured movements of stalking predators or react to fast-moving threats like swooping hawks.
Portrait of a Merlin. All birds have some binocular vision, but this ability is especially important for birds that hunt moving targets at high speeds
Specialized Adaptations in Different Species
Night Vision in Owls
Owls have evolved to hunt after dark using a combination of stealth and wonderfully fine-tuned senses. Their large, tube-shaped eyes are just one of their secret weapons, and they show several specialized traits for operating in low light.
Owls may not have the refined color vision of diurnal birds, but their binocular night vision may be up to 100 times better than pigeons. The secret to excellent low-light sight is a high density of light-sensitive photoreceptors known as rods that comprise about 90% of their retinal receptor cells.
Some authors suggest Owls have relatively poor daytime vision as a trade-off for such amazing eyesight after dark, but owls certainly can see during the day. In fact, Barn Owls and many other species will hunt in broad daylight at some times of the year.
Sharp Vision in Raptors
Owls may have the most light-sensitive eyes, but diurnal raptors have the greatest acuity (sharpness). In fact, the Wedge-tailed Eagle of Australia has the highest resolution of any known animal! Eagles and other raptors owe their powerful vision to their huge, forward-facing, and bifoveate eyes.
Predatory raptors have temporal foveae for forward binocular vision and central foveae for acute lateral vision. Each of these pit-like areas is dominated by the color-sensitive cone cells, which make up about 80% of their photoreceptors.
However, such high resolution means little without good focus, so large birds of prey have evolved the impressive ability to adjust both the shape of the lens and the outer cornea.
Panoramic Vision in Prey Birds
Songbirds and other smaller species lower on the food chain face danger from every direction. They have evolved rapid vision to detect the movements of predators in record time, and their massive field of view allows them to spot a threat from virtually any direction.
Prey birds have eyes positioned on the sides of their head, which allows little visual overlap and, therefore, limited binocular vision. However, lateral placement allows each eye to monitor a large area, and some birds can even see a full 360 degrees around themselves!
While birds with lateral eye placement may have some binocular vision ahead or even behind their head, these species see best with their head turned to the side to use their central foveae.
Barn Owls and many other species will hunt in broad daylight at some times of the year
Mallards can see a full 360 degrees around themselves!
Summary
From Owls that hunt in the dead of night to Woodcocks that have ‘eyes in the back of their head,’ birds have evolved a remarkable variety of adaptations and capabilities to fine-tune their most important sense.
Exploring the hidden secrets of bird anatomy and physiology makes studying our feathered friends so much more fascinating, especially when observing them in nature.
So, the next time you see a soaring hawk, try to picture how it sees you looking back, and keep an eye out for songbirds giving you the side-eye; they’re probably looking straight at you!