Scott K. Johnson
The phrase “every color of the rainbow” isn’t quite as all-encompassing as it sounds. For one thing, the color chips in your hardware store’s paint aisle host some colors you’ll be hard-pressed to point to in a real rainbow. But even on a less hair-splitting level, purple is missing from that rainbow.
The “V” in “ROYGBIV” stands for violet, sure, but that’s not actually the same thing as purple. There is no purple wavelength of light—it requires a mixture of both red and blue wavelengths. That makes it a “nonspectral color”—in fact, it’s the only non spectral color humans see. It requires our brains to interpret signals from both red-sensitive and blue-sensitive cones in our eyes and to see that as a separate color.
But while humans have three types of cones (making us “trichromatic”), many creatures have four, expanding their visible spectrum into ultraviolet (UV) wavelengths. In theory, this means they might be able to see additional nonspectral colors we humans struggle to imagine: UV mixed with either red, yellow, green, or purple. So… do they?
More than just UV
There has been some research on bees demonstrating that they see UV+green as its own color (referred to as “bee-purple”), but there isn’t a whole lot of experimental evidence beyond that. A team led by Princeton’s Mary Stoddard decided to test the idea by taking advantage of hummingbirds’ love of sugar-water feeders.
Working in Colorado over several summers, the researchers set up a pair of feeders for their experiments—one containing that delicious sugar water and one just containing boring old water. On top of each was a special LED light containing UV, blue, red, and green LEDs behind a diffuser, allowing the researchers to light up the feeder in a variety of nonspectral colors.
The researchers watched as wild broad-tailed hummingbirds came to visit, recording which feeder they flew up to first. After a set number of visits, the feeder positions would be switched so the birds couldn’t simply return to the same spot once they found the sweet stuff. The idea was that they would use the color of the light to identify the feeder on return visits. They couldn’t track individual birds separately, but based on some banding, they estimated the local population at 200 to 300 (depending on the year). In total, they recorded over 6,000 hummingbird visits.
The experiments pitted different pairs of colors together. There were a few control runs where both lights displayed the exact same color and a couple experiments testing red vs. green. From there, the differences got more subtle and depended on differentiating nonspectral colors. Most involved different mixtures of UV and another color—in the same way that we could differentiate between a reddish-purple and a bluish-purple.
The tests showed that the birds could see every nonspectral color that the researchers threw at them. Color pairs that were closer together in hue resulted in more mistaken visits but still beat the 50/50 odds of the control experiments.
As an additional plausibility check, the researchers scanned databases of precisely measured colors that appear in plants and birds. These nonspectral colors are quite common in nature, accounting for 30 percent of bird plumage colors and 35 percent of plant colors in the databases. So it would certainly make sense that hummingbirds (and other birds) are able to see these colors in their environment.
And the researchers do think this study is generalizable beyond just the broad-tailed hummingbirds that volunteered for it. Many things are poorly understood about the physiology of eyesight across bird species, much less the neural processing of signals from those color cones in the eye, but what we do know suggests hummingbirds are probably representative. “Although these experiments were performed with hummingbirds,” the team writes, “our findings are likely relevant to all diurnal, tetrachromatic birds and probably to many fish, reptiles, and invertebrates.”
But they also note that it’s hard to get inside these critters’ tiny little heads and understand what this experience is like. “Even if the neural mechanisms for color vision were clear, and even if color-mixing experiments attest to avian tetrachromacy,” they write, “we still could not answer the more philosophical question of what nonspectral colors really look like to birds. Does UV+green appear to birds as a mix of those colors (analogous to a double-stop chord played by a violinist) or as a sublime new color (analogous to a completely new tone unlike its components)? We cannot say.”