FACT: Science Finds Hummingbirds See Ultraviolet Light Invisible to Humans
Hummingbirds can see an impressive array of colors that are invisible—or appear very different—to the human eye, scientists reported June 15 in the journal Proceedings of the National Academy of Sciences. In a series of experiments involving sugar water and LED tubes, the researchers found that wild broad-tailed hummingbirds can discern colors created from various combinations of ultraviolet and visible light. This ability likely helps the birds home in on nectar-bearing flowers covered in patterns that are imperceptible to people.
“Our findings…suggest that these birds really are experiencing a whole range of colors we humans can only imagine,” says Mary Caswell Stoddard, an ornithologist at Princeton University and co-author of the new study. “We humans are really limited in how we understand and appreciate and can describe the color experience of birds and other animals.”
The average human eye can distinguish around one million different colors. Our color vision depends on three types of cones—special cells sensitive to red, blue, or green light. However, many birds, reptiles, and fish have an additional kind of cone that can pick up ultraviolet light.
“If humans are looking at a rainbow, we can see ROYGBIV; a bird looking at the same rainbow would see all those colors in addition to ultraviolet,” Stoddard says. “The second advantage is that the UV cone type should in theory give birds an extra dimension for color perception relative to humans.”
Stoddard and her team wanted to explore which “nonspectral” colors hummingbirds could discern, which they define as colors that an animal sees because their eye's cone cells react to very different parts of the color spectrum at the same time. The only one of these special colors that most humans can see is purple; the “violet” created by a prism or rainbow is technically a kind of blue.
“When we see purple, our blue cones and red cones are being stimulated, but not really our green cones,” Stoddard says.
Hummingbirds made ideal candidates for this investigation because they are sugar fiends that are naturally drawn to brightly colored flowers in search of nectar. For each experiment, the scientists set up two bird feeders—one filled with nutritious sugar water and the other plain water—at Rocky Mountain Biological Laboratory in Colorado. Beside the feeders were two LED tubes, each of which emitted light in a different color.