Vision is the most highly developed and acute bird sense, and birds have a keen sense of color that is vital for finding food, choosing a mate, and more. Understanding how birds see color can help birders take advantage of that sense to better appreciate and attract birds.
What Colors Birds See
Birds see more colors than humans in several ways. Not only are birds able to perceive the familiar rainbow of colors as well as parts of the ultraviolet (UV) spectrum that are invisible to human eyes, but they also have better visual acuity. This means birds can determine subtle differences between similar shades of color, gradations that human eyes are not able to discern. The cells in the eye responsible for color detection, the cones, are in the retina, and birds have four types of cones rather than the three humans have. The exact number of cones varies in each bird species but is typically higher than humans and other mammals.
Each cone in a bird's eye has a drop of oil in it that selectively filters out certain colors, giving birds greater sensitivity to different color shades. This wavelength filtering allows birds to more easily see contrasts in their surroundings, perhaps seeing through the camouflage of insects, prey, or other birds, for example. This type of filtering or polarization is also useful for pelagic birds, allowing them to see deeper into the water than previously believed, which can help them find suitable food sources and prey.
Diurnal birds that are active during the day have the best color sense. Perceiving different colors is less crucial for nocturnal birds, however. Many birds that are most active at night have a greater number of rod cells in their eyes instead, which allows them to capture more light and see better in low light conditions, though they may not see colors as clearly.
Why UV Light Matters
Being able to see UV light is a crucial aspect of how birds see color. For decades, ornithologists assumed that birds saw colors the same way as humans, and many aspects of bird behavior were unexplained until birds' sensitivity to UV light was realized. The ability to see ultraviolet light changes the perception birds have of many objects, even though humans may not see those differences.
- Food: Some berries and other fruits have waxy coatings that reflect UV light, making them stand out vibrantly against green foliage. Birds can see this fruit much more clearly than humans, making foraging much easier. Some insects and flowers also reflect UV light, giving birds a distinct advantage for finding those food sources. This type of vision can also explain why birds may prefer certain types of berries, even though both fruits may be nutritionally similar or ripen at the same times.
- Prey: Raptors use UV light to track prey, since the urine splashes and trails that voles, mice, and other rodents use to mark their territory are brilliantly visible in ultraviolet light. This allows raptors to determine exactly where prey is located for more efficient hunting, even when the prey may not be visible itself.
- Plumage: Species that may not appear dimorphic to humans may actually look very different in UV light. Male blue tits, for example, have a prominent crown seen under UV light, though the genders look similar to humans. Birds have no difficulty telling the difference, and can use UV markings to help select mates, defend a territory, or tell individual birds apart.
- Eggs: Some brood parasite eggs, despite having similar visible colors and markings, look very different from host bird eggs under UV light. This gives nesting birds the ability to tell when an egg is not their own and allows them to reject the foreign egg. While not all species that host brood parasites will reject unwanted eggs, the UV pattern may be a factor for those that do.
Using Birds' Color Sense
Understanding how birds see color is more than just a fascinating field of study for ornithologists. Different birding-related developments have used birds' color sense to enhance equipment and backyard birding tools, such as:
- Adding UV reflective properties to window decals to more effectively prevent bird window collisions without blocking views.
- Designing pet collars and tags that reflect UV light and may make these predators more easily visible to birds.
- Treating field bags or clothing to not reflect UV light, creating more effective camouflage for birding gear.
- Designing bird feeders with UV patterns that can catch birds' interest and entice them to visit new bird feeders more easily.
Much more study is needed to thoroughly understand how birds see color. As each new discovery is made, ornithologists and birders alike can use that knowledge to learn about birds and enjoy them more effectively.