Sense of Wonder: Bird Vision

Despite the title of this article, there is actually no such thing as “bird vision.” We humans naturally think that all other creatures “see” the world as we do. However, each of the approximately 10,000 species of birds has evolved to see the world in a unique way that maximizes its ability to obtain food and avoid predators. This makes a lot of sense when you think of the different visual needs of an American Robin hunting worms in your front yard versus a Peregrine Falcon stooping on an unsuspecting Mallard. Birds embody remarkable differences in visual perception from species to species through the myriad combinations of simple structural differences in their eyes. These structural differences include differences in eye placement, differences in the ability to change the shape of the lens and cornea, and differences in the number, type and distribution of retinal photoreceptors (rods and cones).

Birds have the second-largest eyes relative to body size of all vertebrate land animals (frogs are first). In general, larger eyes see better because they let in more light and have more rods and cones. Birds’ visual field (what they can see of the surrounding world) varies greatly among species based on eye placement in the head. The vast majority of birds have eyes on the sides of their heads that they can point in different directions from each other. Thus, they receive a different image in each eye. Skillfully moving their eyes and heads, they can extract information about the scene receding behind them while at the same time gain information about the world they are traveling into. Humans, on the other hand, have front-facing eyes. This leaves us with the experience that the world is in front of us, and we are moving into it. The American Woodcock even has eyes placed so far towards the back of its head that it can see basically 360 degrees. Try sneaking up on that bird!

Predators like raptors and owls have eyes placed more towards the front of their heads. This gives them better binocular vision, which provides an excellent 3-D image. However, there is evidence that raptors use lateral vision to detect and latch onto prey. Binocular vision is used only when they are very close to the prey.

Birds—especially birds of prey—have a limited ability to rotate their eyes. Therefore, they are constantly shifting their head position to obtain different images in their eyes. Next time you see a hawk perched in a tree, notice how it constantly shifts its head to obtain different images and detect prey.

Birds have muscles surrounding the cornea and the lens. These muscles allow birds to change the shape of the cornea and lens, which changes the focus of the image as it travels through the eye before striking the eye’s retina (humans can only change the shape of our lenses, not our corneas). From the retina, the image is transmitted to the brain for analysis. Bird species have differing abilities to change the shape of the cornea and lens and therefore the image that arrives at the retina. This means that two different species looking at the same scene will present different images to the retina for analysis.

Human retinas have one type of rod and three types of cones. Rods handle black and white vision. Cones handle color vision. Humans can differentiate roughly one million colors. Many bird species have an additional fourth type of cone. The fourth cone allows them to see many more colors than humans, including colors in the ultraviolet range. The advantage of vision in the ultraviolet range is that many flowers, fruits, berries and bird feathers reflect ultraviolet light, making them easier to detect. There is even some controversial evidence that the Eurasian Kestrel is able to follow the urine trail of its prey, the vole, because the urine reflects ultraviolet light.

The number and distribution of rods and cones in bird retinas has a great impact on their visual acuity. Birds have 2-5 times the number of cones per square millimeter when compared to humans. This gives them a visual acuity 2-8 times better than humans. Some bird species, including raptors, kingfishers and hummingbirds, have two foveae. Foveae are the areas of the retina packed with the greatest density of photoreceptors, which produce the sharpest vision. This gives these species acute forward-facing and side-facing vision. Humans have only a single fovea. There are even bird species such as some cockatoos and corellas that have photoreceptor densities that differ between the eyes. Many of these species have greater densities in the left eye and therefore use their left foot and eye to pick up and inspect objects.

Birds see the world very differently than we do. In addition, each bird species sees the world in a uniquely different way from other species. So this article should properly be titled “Bird VisionS.” You can help with those “visions” for night-migrating birds by reducing the problem of light pollution.

Natural selection has created a complexly wonderful range of visual capabilities. The next time you are watching a robin hunt a worm in your yard, think about the way the robin sees the worm brightly fluorescing in the sun’s ultraviolet light, with incredibly fine detail on its surface. It may still taste like a worm—but wow, what a different visual experience!

Kevin Browngoehl, Birding Club of Delaware County Member

Photo credits:

American Woodcock, by Fyn Kynd, licensed under CC BY-SA 4.0
American Robin, by Melahi3.14, licensed under CC BY-SA 4.0