Hummingbirds can see up to five "colors" that humans are unable to perceive. 

New research led by Princeton University has trained wild hummingbirds to take part in a number of color vision experiments and found the species is able to pick up on “non-spectral” color combinations that humans can't see. 

Human vision, like most other primates, is trichromatic and uses three types of photoreceptor cells (known as cones) found in the eye: blue cones, green cones, and red cones. Reported in the journal PNAS, the new research shows that hummingbirds instead use an ancient four-color cone visual system that allows them to see ultraviolet non-spectral color combinations that are not available to humans.

It’s not easy to explain how these colors would appear to humans as something we are incapable of perceiving, but “non-spectral” colors are colors not on the spectrum of visible light. They are made up of combinations of hues from widely separated parts of the color spectrum, such as magenta, made up of blue and red wavelengths.

Most colors we perceive are a blend of neighboring colors, such as teal (blue-green) or yellow (green-red). However, magenta is somewhat comparable to a “non-spectral” color combination as it's not strictly a color of the rainbow, nor a blend of neighboring colors, but our blue and red cones are stimulated. While humans can perceive just one “non-spectral” color combination, hummingbirds can theoretically see up to five: purple/magenta, ultraviolet+red, ultraviolet+green, ultraviolet+yellow, and ultraviolet+purple.

“Humans are color-blind compared to birds and many other animals,” Mary Caswell Stoddard, an assistant professor in the Princeton University Department of Ecology and Evolutionary Biology, said in a statement. “Humans have three types of color-sensitive cones in their eyes — attuned to red, green and blue light — but birds have a fourth type, sensitive to ultraviolet light. 

“Not only does having a fourth color cone type extend the range of bird-visible colors into the UV, it potentially allows birds to perceive combination colors like ultraviolet+green and ultraviolet+red — but this has been hard to test,” continued Stoddard.

To reach these findings, the team headed to the Rocky Mountain Biological Laboratory in Gothic, Colorado. They set up a number of different bird feeders – some empty, others filled with sugar water – and placed next to them LED lights. Some emitted “normal” spectral colors and others non-spectral colors.

After wild broad-tailed hummingbirds (Selasphorus platycercus) learned which feeders contained food, the researchers periodically swapped the positions of the feeders, so the birds could not simply use location to remember where the rewards were. By tracking over 6,000 feeder visits, they concluded that the hummingbirds clearly knew which feeders contained food by identifying them by non-spectral color.

“It was amazing to watch,” explained Harold Eyster, a PhD student from the University of British Columbia and a co-author of the study. “The ultraviolet+green light and green light looked identical to us, but the hummingbirds kept correctly choosing the ultraviolet+green light associated with sugar water.”

"It is impossible to really know how the birds perceive these colors. Is ultraviolet+red a mix of those colors, or an entirely new color? We can only speculate,” added Ben Hogan, a postdoctoral research associate at Princeton and a co-author of the study.

Hummingbirds aren't the only creatures that have four different types of color cone cells. It's a visual system that's also widely seen in birds, many fish and reptiles, and most likely many species of dinosaurs.