US, WASHINGTON (ORDO NEWS) — Results from experiments with tri-color Selasphorus platycercus in Colorado suggest that the ability to distinguish between non-spectral colors (including ultraviolet waves) can play a vital role in the behavior of this species of hummingbird, including mating, feeding and avoiding predators.
Unlike people who have three types of color-sensitive cones in their eyes, birds have four types of cones that help them handle differences between different colors. With three cones, human eyes can perceive the so-called trichromatic color, consisting of a neural mixture of red, green and blue light.
Thanks to this, our brain can perceive, for example, such a non-spectral color as violet (because it is a combination of blue and red). But animals with an extra cone can see an even wider range of colors, being sensitive to more light wavelengths – opening the door for other types of color combinations that we cannot see or even imagine.
People are color blind compared to birds and many other animals. Not only does the fourth type of cone expand the range of colors visible to birds to ultraviolet radiation, it potentially allows birds to perceive combined colors such as ultraviolet + green and ultraviolet + red – it was difficult to verify, ”Mary Caswell Stoddard, evolutionary biologist at Princeton University.
It is believed that birds can perceive up to five non-spectral colors, including violet, ultraviolet + red, ultraviolet + green, ultraviolet + yellow, and ultraviolet + violet.
To test this, Caswell and her team installed bird-eye LED tubes that were programmed to display a range of colors, including non-spectral colors that people cannot see. These devices were then placed next to water bowls, some of which contained sugar water (which birds like) next to one color, while others contained plain water next to another color.
Then they swapped these feeders to find out if birds can use the color indicator to determine where what water is. In a series of randomized experiments over three years (including thousands of feeding sessions), scientists found that tricolor selasphorus can see combinations of ultraviolet colors that are inaccessible to humans.
As it turned out, the birds could easily distinguish between different types of non-spectral colors to get a more coveted reward.
“The ultraviolet + green light and the green light looked identical to us, but the hummingbirds continued to choose the right ultraviolet + green light associated with sugar water. Our experiments allowed us to take a look at how the world looks like a hummingbird, ”said Harold Eister, co-author of the study at the University of British Columbia.
In another experiment, the researchers analyzed nearly 1,000 different types of plumage of birds and almost 2,400 different species of plants and found that about a third of all colors were perceived by birds as a non-spectral color. This indicates how much visual power can affect the interaction of birds with the world.
“These results are consistent with the assertion that birds are tetrachromats, so the color space of birds represents a wide range of behavioral and environmentally significant colors, many of which people (or any trichromat) cannot even imagine,” the researchers explain in their article.
However, the authors of the work indicate that their study cannot serve as guaranteed evidence of the tetrachromatic visual system possessing birds. In the future, more rigorous experiments with more colors may help us further understand the limits of color perception by birds. But even then we will still not know much about what nervous mechanisms allow the brain of animals to perceive these colors.
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