Hummingbirds Able to Perceive Expansive Array of Colors Humans Can Not

Researchers trained wild hummingbirds, like this male broad-tailed hummingbird, to perform a series of experiments that revealed that the tiny birds also see combination colors like ultraviolet+green and ultraviolet+red. (Photo courtesy of Noah Whiteman)

(CN) — Hummingbirds may be largely known for their furiously quick-flapping wings and avid love for sugar, but new research released Monday shows they have earned another claim to fame: an ability to see colors the human eye could only dream of.

It has been well established that birds have evolved into incredibly visual creatures. Their amazing eyesight gives them the ability to pinpoint food from far away, escape the clutches of dangerous predators and even impress potential mates. Compared to some of these highly visual animals, researchers say, human eyesight pales embarrassingly by comparison.

This avian vision is partially due to the fact that birds have an additional visual cone nestled within their eyes. While humans have three color cones – one each to see red, green and blue light – birds are equipped with a fourth cone that gives them the ability to see ultraviolet light.

Researchers have suspected this extra color cone may give birds the ability to see beyond just the ultraviolet spectrum, as this extra visual tool may also help them see color combinations humans simply cannot observe.

One example of this that researchers were particularly interested in is a bird’s ability to see nonspectral color, a color combination that is created from colors that came from different parts of the color spectrum.

While humans largely have just one nonspectral color, purple, birds potentially have five: purple, ultraviolet+red, ultraviolet+green, ultraviolet+yellow and ultraviolet+purple.

Experts say that these theories, however, has proven somewhat difficult to test.

This all changed when researchers announced in a new study published Monday in Proceedings of the National Academy of Sciences that were able to devise an experiment that clearly demonstrates that hummingbirds are cable of seeing a spectrum of colors and color combinations that are completely unperceivable to the human eye.

Scientists from myriad institutions, including Princeton, the University of British Columbia, Harvard University, University of Maryland and the Rocky Mountain Biological Laboratory, began the experiment by creating a series of special LED tubes that were built to showcase wide range of colors, including the nonspectral colors humans can’t see.

Researchers then took these “bird vision” tubes to a high-elevation alpine meadow that was regularly inhabited by broad-tailed hummingbirds.

Mary Caswell Stoddard, an assistant professor in the Princeton University Department of Ecology and Evolutionary Biology and first author of the study, said that the natural evolutionary traits and lifestyles of hummingbirds make them exceptional candidates for vision experiments such as this, particularly when compared to other avian species.

“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 with the release of the study. “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.”

Every morning before the sun crested the horizon, researchers would set up two feeders in the wild and pair each feeder with one of these LED tubes. One feeder contained the kind of sugar water hummingbirds are so fond of and the other contained only plain water, while the feeder containing the sweet water would be paired with an LED tube displaying a color different than the color paired with the plain water.

After recording over 6,000 feeder visits across 19 experiments, including control experiments to ensure that smell and location tactics were not playing a part in the birds’ decision making, researchers found that the birds learned to flock to the feeder paired with the color that contained the sugar water.

These findings demonstrated that birds were able to distinguish between pure colors and ultraviolet combination colors. For example, birds were able to easily tell the difference between pure green and ultraviolet+green – colors that look exactly the same to the human eye.

“It was amazing to watch,” Harold Eyster, a UBC Ph.D. student and one of the co-authors 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.”

Researchers also found that hummingbirds could also perceive these nonspectral colors in nature. After analyzing data taken from over 3,000 feather and plant colors, researchers found that hummingbirds could see many of the colors in a nonspectral fashion while humans could not.

Anybody who is holding out for a scientific breakthrough that could allow humans to see these colors firsthand, however, are encouraged not to hold their breath.

“Could we engineer goggles to allow humans to see some of these colors,” Stoddard said in an email. “I wish! The challenge is that color perception is complicated, requiring neural comparisons of photoreceptor outputs. Even UV-detecting goggles would not allow humans to see a color like UV+green like birds do.”

With these restrictions, researchers note there is no way of knowing how birds actually see these colors. When a bird sees a color paired with ultraviolet, such as ultraviolet+green, it is virtually impossible to tell if they are seeing just a strange mix of those two colors or perhaps a new color entirely.

Humans, equipped with such drastically limited eyes when tested against those of our avian contemporaries, are left to only guess.

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