Hummingbirds can perceive colors that humans cannot detect, according to a team of researchers from Princeton University.
According to Caswell Stoddard, an assistant professor in the Princeton University Department of Ecology and Evolutionary Biology and the lead author of the study, humans are "color-blind compared to birds and many other animals."
Humans are trichromatic: our eyes have three types of color-sensitive receptors or cones: blue, green, and red.
Hummingbirds, on the other hand, are tetrachromatic, which means they have four color cones. This feature is necessary for them to survive as they rely on color vision in searching for food, mates, escaping from predators, navigating through terrains, among others.
As human's eyes are trichromatic, we can only perceive the rainbow colors and a pure non-spectral color: purple. Humans can see purple as it stimulates the red and blue cones simultaneously.
The color cones of the birds, on the other hand, allows them to see a broader range of colors. These colors include the ultraviolet spectrum, which provides for UV-green and UV-red.
The researchers from Princeton University were able to illustrate these facts when they carried out a field experiment with wild broad-tailed hummingbirds near the Rocky Mountain Biological Laboratory in Colorado.
The study showed that birds could tell the difference between spectral-colored feeders from non-spectral ones.
"Seeing them do this right in front of my eyes is one of the most exciting things I've ever witnessed," Stoddard admitted.
The results of the study were published in the journal Proceedings of National Academy of Sciences this week.
Birds Can See Colors That Humans Cannot
To test if hummingbirds can indeed see non-spectral colors, the research team set-up a field system for the birds to explore bird color vision in a natural setting at Rocky Mountain Biological Laboratory (RMBL) in Gothic, Colorado.
Wild broad-tailed hummingbirds (Selasphorus platycercus) were trained for this color vision experiments. Hummingbirds, according to Stoddard, are perfect participants for this experiment because they prefer sugars and can respond to flower colors that show nectars rewards. This makes them learn color association quickly with little training, Stoddard said.
A customized "bird vision" LED tubes that display a broad range of non-spectral colors were made for this study. Researchers set up feeders containing sugar water or just plain sugar and placed an LED tube beside the feeder every day. The tube beside the sugar water had one color, while the plain water had another different color.
The study revealed that wild hummingbirds tended to visit rewarding colors and perceive various non-spectral colors.
Harold Eyster, a UBC Ph.D. student and co-author of the study, revealed that it was amazing to see the birds choose the ultraviolet colors associated with sugar water every time. The ultraviolet+green light and green light seemed identical to the researchers, but the hummingbirds kept choosing the ultraviolet combination that is associated with water.
The research team also found out that the birds perceive the 3,315 feathers and plant colors as non-spectral colors. Humans, on the other hand, sees it differently.
Having four color cones like the hummingbird's evolved in early vertebrates. The said color vision is common for birds, fish, reptiles, and dinosaurs, Stoddard explained.
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