Avian Eye Structure
Birds have special eyes. They rely on their vision for everything from weaving between obstacles in flight to snatching insects from the air or fish out of the water. Birds also use their eyes to spot predators, and the difference between plumage colors can tell them a lot about the quality of a potential mate.
Bird’s eyes are unique in many ways, and their incredible eyesight is just one of the many adaptations that have made them the most diverse group of land vertebrates. In this guide, we’ll take a closer look at the avian eye and some of its most remarkable structures.
Sizing Up the Avian Eye
Proportional Importance
Most birds have horizontally flattened eyes, while some have more globe-shaped eyes. Owls, which have remarkable night vision, differ in having tube-shaped eyes. Regardless of shape, all birds have very large eyes in proportion to their bodies.
The relative size of a bird’s eyes differs dramatically from a human’s, and Owl eyes may account for a whopping three percent of their body weight. A bird also holds the title for the largest eyes of any land animal. The Ostrich has snooker ball-sized eyes that are even bigger than their brains!
Placement
The position of birds’ eyes varies depending on both their diet and their place on the food chain. Hawks and Owls, for example, have eyes on the front of their face. Forward-facing eyes create a significant overlap in the light captured by the left and right eye, which combines to create binocular or stereoscopic vision. This helps them track their prey and provides crucial depth perception when going in for the kill.
Most birds have eyes located on the sides of their head. This placement limits their binocular vision but allows them to see in a much wider arc around their heads. In fact, birds like Pigeons can see over 300 degrees, and Mallards can see a full 360 degrees on the horizontal plane.
Birds also need to see upwards and downwards to detect food or a lurking predator like a coiled snake on the ground or a hawk circling above. Remarkably, the Woodcock can see almost 180 degrees on the vertical plane. Combined with their 360-degree horizontal vision, these big-eyed birds are pretty tough to sneak up on!
Woodcocks can see almost 180 degrees on the vertical plane
Key Components of the Avian Eye
Cornea and Iris
The cornea is a transparent covering over the front of the eye that consists of several layers. It does not contain blood vessels and functions to refract and direct light into the eye and to support the eyeball.
The bird’s iris controls the amount of light entering the eye. This structure comes in a surprising variety of colors, from the bright red of the Black-crowned Night Heron and the Coot to the bright yellow of the Cuckoo and the Great-horned Owl.
The endangered Bank Cormorant of Africa even has a multi-colored iris. The function of these colors is not always clear, but bright irises may help attract a partner in the same way that colorful plumage does.
Pupil
The pupil is the dark central part of the visible eye that is surrounded by the iris and covered by the cornea. Light must pass through the pupil to reach the retina, and since more light is needed under dim conditions, a bird’s pupil appears larger when it's dark. The same thing happens to us when our eyes ‘adjust’ at night.
Retina
The retina is the part of the eye that contains photosensitive structures called rods and cones. Rod cells are light-sensitive and more dominant in nocturnal birds like Owls that need good low-light vision. Cone cells are used for daylight color vision and are the dominant photoreceptor in most bird species.
Types of Cone Cells
Birds exhibit tetrachromacy, which means they have four types of color-sensitive cones in their eyes. Like us, birds have red, green, and blue-sensitive cones, but they also have the remarkable ability to see ultraviolet light. Each of their four color-sensitive cone cell types contains oil droplets of different colors that filter light as it passes through.
Fovea
Birds also have special areas within the retina called foveas that provide exceptionally high visual acuity. These pit-like structures contain high concentrations of photoreceptors packed about five times as densely as in our own eyes. Avian foveas are thought to provide the highest resolution but also, perhaps, to enlarge the image.
Most birds have a single fovea on each retina, but some birds, like Kingfishers and Eagles, have two in each eye. A study on the fovea of birds of prey indicates that their shallow foveas improve close-range vision and are used when the bird looks straight at their target. The deep foveas are more powerful and used when looking at targets at longer distances. However, the bird must turn its head at about 45 degrees to use these special areas.
Pecten Oculi
Birds have a unique structure within their eye known as the pecten oculi. This pigmented organ projects outwards from the retina near the optic nerve in a position that doesn’t impair the bird’s vision. Its function is not fully understood, although a few possibilities have been suggested.
The pecten is rich in blood vessels, which could indicate that it functions to supply oxygen and nutrients to the retina, which does not have its own blood supply. It is also suspected to maintain the pH of the fluid in the eye.
The Black-crowned Night Heron has bright red eyes. The bird’s iris controls the amount of light entering the eye
Specialized Lenses and Focus
Flexibility of the Lens
Light passing through the bird’s pupils must travel through the lens before reaching the photoreceptors on the retina. The avian lens is a soft, transparent structure that can be adjusted to focus the image depending on its distance.
The process of changing the lens shape to focus vision is known as accommodation, and birds have some interesting mechanisms for getting a clearer picture. Read on to learn more about this vital function.
Accommodation
Birds need to focus really fast when they’re in flight, especially when catching live prey or avoiding fast predators. They do this by adjusting the shape of their lens with their sclerocorneal muscles and ciliary body.
Unlike mammals, birds can also adjust the shape of their cornea, and diving birds can even squeeze the lens with their iris, forming a bulge that compensates for the effects of water refraction.
The Protective Mechanisms
Eyelids and Nictitating Membrane
Birds’s eyes are exposed to many dangers as they fly and forage, so they rely on eyelids and nictitating membranes to protect their corneas from dust, mud, and other foreign matter that could affect their vision.
Like us, avians have an upper and lower eyelid that can be raised and lowered, although the lower lid shows the most movement in birds. These eyelids provide maximum protection and are closed when the bird sleeps.
Birds also have an eyelid-like structure known as the nictitating membrane. This transparent or translucent membrane lies beneath the eyelids and closes over the eye horizontally or diagonally to clean, protect, and moisten the cornea.
Protective Glands
Birds need to keep the surface of their corneas moist and clean for a clear view of the world. They have harderian and lacrimal glands that secrete fluids to lubricate and protect the eye. The harderian glands secrete a thick, oily substance, while the lacrimal glands secrete tears.
Interestingly, the oily secretions of the harderian glands have been shown to trigger an immune response against pathogens and protect the bird against infections.
Portrait of a Cockatiel sleeping. Like us, avians have an upper and lower eyelid that can be raised and lowered
Summary
With such a fast-paced lifestyle, birds need fast and powerful eyes to stay fed and keep themselves out of trouble, and nature has provided just that through the process of evolution.
Birds’ eyes are large, sharp, and complex structures with advanced mechanisms and photoreceptors that work in symphony for receiving and modifying light for visual processing.
We may never know exactly how birds perceive the world, but one thing’s for sure: they have amazing eyes!