Hummingbirds' vision hints at compound colors outside normal spectrum

This is an Inside Science story.

Hummingbirds can see colors humans can only imagine, an ability that now sheds light on an extra dimension of animal vision, a new study finds.

Compared to many other animals, humans are colorblind. Whereas humans possess three kinds of color-sensitive cones -- eye cells sensitive to red, green and blue light -- birds have a fourth cone type sensitive to ultraviolet light.

In theory, this extra cone type could help birds see far more colors than we can. Humans can perceive not only spectral colors -- those made up of a single sliver of the visible spectrum of light, ranging from red to violet -- but also non-spectral colors absent from the rainbow. One example is purple, which results from mixing red and blue, colors from opposite ends of the spectrum. Items we see as purple involve light rays that simultaneously activate red and blue cones in our eyes. Likewise, scientists had thought birds might perceive a mixture of ultraviolet and red as its own color, whereas humans would just see red.

To see if birds can perceive additional non-spectral colors, scientists experimented with wild broad-tailed hummingbirds in an alpine meadow in Gothic, Colorado, which the birds frequented during breeding season. The researchers built LED tubes that could display a wide range of colors, including combinations involving ultraviolet light. Before dawn each summer for three years, they set up two feeders -- one with sugar water, the other plain water -- and placed an LED tube beside each feeder so the hummingbirds could learn to visit rewarding colors.

Based on more than 6,000 feeder visits, the scientists found hummingbirds can see not only purple, but also ultraviolet plus green, ultraviolet plus red and ultraviolet plus yellow. They could also tell apart different mixtures of ultraviolet and red light as well as ultraviolet and green light. "It was thrilling to watch hummingbirds learn to discriminate two different colors, such as ultraviolet plus green and green, respectively, that appeared identical to us," said study lead author Mary Caswell Stoddard, an evolutionary biologist at Princeton University.

After analyzing 965 feather colors and 2,350 plant colors, the researchers suggested birds likely perceive not only the ultraviolet rays invisible to humans, but also many more unique non-spectral colors that we cannot see. This suggests these additional non-spectral colors might prove vital in nature for everything from scrounging for food to wooing a mate.

Tetrachromacy -- the possession of four cone types -- appeared early in vertebrate evolution, and is common in birds as well as in many reptiles and fish, and likely even dinosaurs, Stoddard said. As such, these new findings may carry broad implications for understanding vision across the animal world, the scientists noted.

The researchers detailed their findings online June 15 in the journal Proceedings of the National Academy of Sciences.

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