A Bird's Eye View: Ultraviolet Vision
By Jennifer Meikle, Environmental Educator
To us humans, male and female birds can look identical, like the Cedar Waxwings pictured above. But there is more going on than meets the eye – our eye, specifically. Eyes contain cone cells which are sensitive to color and light. We have three cone cells in our eyes that allow us to see light ranging from 400 to 700 nanometers, while many birds have an additional cone that allows them to see from 320 to 400 nanometers. Their cones also contain tiny oil droplets that help protect their eye from excess UV and ignore unnecessary wavelengths. This means that they can detect UV light, see more colors, and more easily distinguish between similar colors than humans. Researchers have found that many birds plumage reflects UV light. While to us the male and female Waxwings look exactly the same, through a bird’s eye there is no way the two sexes could be confused. Cedar Waxwings are not the only case of this; many other birds previously thought to show sexual monomorphism (male and females that look the same) rarely actually do.
This ability to see UV light and see more colors and wavelengths of light affects more than just mate choices. It makes an impact on many different aspects of life for a bird. Brown-headed cowbirds and cuckoos will leave their eggs in the nests of other species so that they don’t have to use their energy to build a nest and raise their own babies. Some birds may be able to tell the difference between their eggs and the eggs of a parasitic bird through UV light, which causes them to reject the parasitic egg even though the two eggs might look indistinguishable to a human. It also comes into play after the eggs hatch. The bright colors of the edge of the mouth or head of a baby bird signals to their mom and dad to feed them more. UV signals might make this even more pronounced.
Birds also use their UV vision to help find food. Many insects have a body coating that reflects UV light. Seeds, berries, and fruits also develop a highly reflective coating as they ripen. While fruits and berries are already pretty bright to us, they stand out even more to a hungry bird, especially because the background of green leaves does not reflect much UV light making the color contrast even more obvious. Other pollinators like bees can also detect UV light, which makes the reproductive areas of the flowers stand out. A buttercup looks uniformly yellow to us, but to a bee the center is darker where it reflects UV light, and it makes it much easier for pollinators to find.
Really imagining what a bird sees seems impossible to fully grasp, similar to imagining how a bat hears or how whales communicate. We can guess that because they can see a wider range of colors and light than us, what they see is probably lot more varied. With UV detection, already bright colors probably look even brighter!