Compared to many birds, such as the delightful hummingbird (Colibri), humans are color-blind, says Mary Caswell Stoddard. The Princeton University professor, along with colleagues, showed that hummingbirds are able to discriminate various ultraviolet (UV) color combinations, allowing the birds to see the world in additional colors that humans can’t even comprehend.

A hidden world of ultraviolet

Humans essentially see the world in a combination of three colors: red, green, and blue. Each primary color is detected and decoded by corresponding specialized cones in the eye.

The hummingbird, however, has a fourth color cone, which extends its color-vision range into the ultraviolet. But, how exactly does this additional color cone morph the bird’s vision?

In their new study, Stoddard and colleagues left their labs at Stanford and traveled to Gothic, Colorado, for fieldwork in the alpine meadows each summer over the course of three years.

Their work mainly focused on how hummingbirds sense non-spectral colors, which are combinations of hues from widely separated parts of the color spectrum. A clear example of a non-spectral color is purple, which combines blue and red wavelengths of light, but not green. In contrast, teal (blue-green) and yellow (green-red) are blends of neighboring colors in the spectrum.

In fact, purple is the only non-spectral color that humans can sense. But birds should theoretically be able to see up to five, thanks to their extra color cone type. In addition to purple, birds should also be able to see combinations of ultraviolet and red, green, yellow, and purple, respectively.

“Most detailed perceptual experiments on birds are performed in the lab, but we risk missing the bigger picture of how birds really use color vision in their daily lives,” Stoddard said in a statement. “Hummingbirds are perfect for studying color vision in the wild. These sugar fiends have evolved to respond to flower colors that advertise a nectar reward, so they can learn color associations rapidly and with little training.”

The research team performed a series of experiments with wild broad-tailed hummingbirds (Selasphorus platycercus) that had two feeders at their disposal that they could use. One contained sugary water, the other just plain water.

Each feeder had an LED tube that flashed various colors. The tube that corresponded to sugar water emitted one color, while the other emitted a different color — with an important caveat.

To you or I, both colors look the same, i.e. green. However, the LEDs can display a broad range of colors, including non-spectral colors like ultraviolet plus green. If the birds could indeed see additional non-spectral colors, this should be obvious from their choice of feeders.

The researchers swapped the positions of the rewarding and unrewarding tubes at random intervals of time to make sure the birds weren’t going to the same location to pinpoint the treat. They also performed control experiments to rule out the influence of smell or anything other non-vision-related cues.

Over the course of 19 experiments, the researchers recorded over 6,000 feeder visits. An analysis of the hummingbirds’ feeding patterns showed that the birds can distinguish ultraviolet plus green from pure ultraviolet or pure green, as well as two different blends of ultraviolet plus red light (one redder, the other less so).

“It was amazing to watch,” said Harold Eyster, a University of British Columbia Ph.D. student 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. Our experiments enabled us to get a sneak peek into what the world looks like to a hummingbird.”

You might be curious to learn what these additional colors look like. Unfortunately, there is no way to see them — we simply lack the hardware to do so due to the absence of the fourth color cone type.

“The colors that we see in the fields of wildflowers at our study site, the wildflower capital of Colorado, are stunning to us, but just imagine what those flowers look like to birds with that extra sensory dimension,” said co-author David Inouye.

No need to be too jealous, though. The researchers also analyzed a dataset of 3,315 feather and plant colors and found that birds likely perceive many of these colors as non-spectral. However, these non-spectral colors do not stand out relative to other colors that are also visible to humans.

The findings appeared in the journal Proceedings of the National Academy of Sciences.