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June 15 (UPI) -- To find nectar-rich flowers and spot the iridescent signatures of would-be mates, hummingbirds must be able to see and recognize a wide variety of colors. According to a new study, published Monday in the Proceedings of the National Academy of Sciences, hummingbirds can even see a diversity of so-called nonspectral color combinations.
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"When we say nonspectral, we mean a combination color that arises when the color cones are stimulated by light from widely separated parts of the color spectrum," Mary Caswell Stoddard, lead author of the new study and an assistant professor of ecology and evolutionary biology at Princeton University, told UPI in an email. "For humans, purple is the clearest example of a nonspectral color."
Thanks to a breakthrough series of perceptual experiments, scientists confirmed the ability of hummingbirds to see a range of nonspectral colors, including nonspectral combinations involving visible and ultraviolet light.
Scientists have documented the ability to see ultraviolet light in hundreds of species, including birds. Some of the earliest studies of UV vision in birds involved hummingbirds.
In addition to the three color cones possessed by humans -- red, green and blue -- hummingbirds boast a fourth cone, sensitive to UV light. Until now, scientists had yet to show how hummingbirds use their four-cone vision, or tetrachromacy, to perceive combination colors like UV+green and UV+red.
"Even though biologists have long assumed that birds can discriminate a variety of diverse nonspectral colors, our results confirm that this is indeed the case for hummingbirds," Stoddard said. "Personally, it was thrilling to watch hummingbirds learn to discriminate two different colors -- such as UV+green and green, respectively -- that appeared identical to us."
Most perceptual experiments involving birds are carried out in labs, but for the new study, scientists at Colorado's Rocky Mountain Biological Laboratory trained wild broad-tailed hummingbirds, Selasphorus platycercus, to participate in a series of color vision tests.
Using special UV light tubes, researchers produced different nonspectral combination colors at hummingbird feeders with sugar water and feeders with plain water.
"We periodically swapped the positions of the rewarding and unrewarding light tubes, so that the birds could not simply memorize the location of the sweet treat," Stoddard said.
Scientists tracked feeding visitation patterns and determined that the wild hummingbirds learned to differentiate between the nonspectral combination colors. Control experiments confirmed the birds relied on their vision -- and not their smell or some other sense -- to target the feeder with sucrose.
At present, humans can only imagine what exactly UV+green and UV+red looks like, but scientists are working on visualizing the eyesight of hummingbirds.
"In my lab, we obtain UV images and write software to help translate the bird visual experience into something we can appreciate," Stoddard said. "Ultimately, we have to translate some colors, like many bird nonspectral colors, into something humans can understand, because we cannot fully know what those colors really look like to birds."
Though scientists suggest their unique LED light tubes and novel experimental design could be used to study UV vision in other bird species and wild animals, Stoddard and her colleagues plan to continue investigating nonspectral color perception in hummingbirds.
"Now that we have established a system for studying hummingbird color vision in the wild, we are excited to test additional questions," Stoddard said.
"Do all individual hummingbirds learn at the same rate? Is learning affected by how many colorful flowers are in bloom? Can hummingbirds discriminate different spatial patterns? We also want to study how hummingbirds use colors in their daily lives -- visiting flowers and performing courtship displays," Stoddard said.
