06/27/2020
To them, the sky is purple
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What Colors Birds See
Birds see more colors than humans in several ways. Not only are birds able to perceive familiar colors as well as parts of the ultraviolet spectrum that are invisible to human eyes, but they also have better visual acuity to determine subtle differences between similar shades of color, gradations that humans are not able to discern.
The cells in the eye responsible for color detection – cones – are positioned 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.
Diurnal birds that are active during the day have the best color sense. Perceiving different colors is less crucial for nocturnal birds, and 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 as humans, and many aspects of bird behavior were not able to be explained 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 the fruit much more clearly, making foraging much easier. Some insects also reflect UV light, and certain flowers will as well, giving birds a distinct advantage for finding those food sources.
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 t**s, 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 those 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, giving birds the ability to tell when an egg is not their own and allowing them to reject the interloper. While not all species that host brood parasites will reject unwanted eggs, the UV pattern may be a factor for those that do.
Wavelength Filtering
Not only can birds see ultraviolet light, but they see visible colors more distinctly than humans can. 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 allows birds to more easily see contrasts in their surroundings, perhaps seeing through the camouflage of 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.
