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New Research Shows That Hummingbirds Can See Colors We Can’t Even Imagine

by DailyHealthPost Editorial

The light spectrum is responsible for a lot of things. The colors we see are entirely dependant on different wavelengths of light being reflected off of surfaces. Things we see as “green” appear so because they reflect green light and absorb the rest of the visible lights, for example. 

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It’s important to note that what we refer to as “visible light” or the “visible light spectrum” is only what’s visible to us. There are many other animals, birds, and reptiles that are actually able to see more colors than we do. A recent example of that are hummingbirds which were found to perceive at least five additional colors than what humans can see. The interesting findings were published in a report in the PNAS journal.

The reason that’s possible is that different animals have different eyes. Our vision, like that of most other primates, is called “trichromatic” or “tri-colored”. That’s because our eyes use three types of photoreceptor cells (called “cones”) – blue cones, red cones, and green (not yellow) cones. The difference between us and hummingbirds is that they have four such “cones” in their eyes, allowing them to see colors outside of our “visible light spectrum”.

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Of course, it’s difficult for us to imagine what these colors look like to birds. That’s why we call these colors “non-spectral”. As best as we can guess, these colors are likely made out of hues from widely separated parts of the visible spectrum that we usually can’t perceive. Maybe they are similar to magenta which is made out of both blue and red wavelengths? 

Most of the colors we can see are made up of blends of “neighboring” colors on the spectrum. Yellow is a mix of red and green, teal is a combination of green and blue, and so on. In that sense, magenta is one of the rare “unique” colors we can see because red and blue are not neighboring colors. Therefore, the assumption is that some of the “extra” colors animals such as hummingbirds can see are other such invisible to us combinations of non-neighboring basic colors.

The colors tested by the researchers included combinations of ultraviolet plus green, ultraviolet and yellow, ultraviolet and purple, ultraviolet and red, and purple plus magenta. All five of these combinations are unrecognizable to the human eye but they seem to have caught the hummingbirds’ attention.

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Humans are color-blind compared to birds and many other animals,” explained Mary Caswell Stoddard, an assistant professor in the Princeton University Department of Ecology and Evolutionary Biology. In her statement she also elaborated that “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.”

The research team from the Rocky Mountain Biological Laboratory in Gothic, Colorado had a pretty extensive test to arrive to these findings. They set up multiple different bird feeders and filled only some of them with sugar water. Then, they placed LED lights over each of the bird feeders, with each light going off in a different color combination – some “normal”, i.e. in the visible spectrum, and others in non-spectral colors.  

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Once the wild broad-tailed hummingbirds (Selasphorus platycercus) they were working with identified the feeders with sugar water, the scientists started switching things up. Every once in awhile they’d move the bird feeders around while still keeping the corresponding LED lights with them. 

The result? The hummingbirds kept honing into the “correct” lights and finding the full bird feeders without an issue. This led the RMBL researchers to the conclusion that the birds didn’t use their special orientation to find the bird feeders but instead, the “invisible” colors provided by the LED lamps.

“It was amazing to watch,” said Harold Eyster, co-author of the study and a Ph.D. student from the University of British Columbia. “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.”

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“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,” said Ben Hogan, also a co-author of the study and a postdoctoral research associate at Princeton.

Four-coned eyes aren’t common to just hummingbirds. They are also present in many other birds, reptiles, and fish. This likely means that they were also the norm for dinosaurs.

Something that’s significantly rarer are pentachromacy eyes – these are eyes with five different photoreceptor cells or cones. Pentachromacy sight allows the user to see even more differing ranges of the wavelight spectrum. Needless to say, these are even more unimaginable to us. Pentachromacy is seen in some bird species such as pigeons as well as in butterflies. Lampreys (a type of Petromyzontiformes fish are also believed to have pentachromacy sight but there’s still much more research in that area to be done.

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