A new study, published in the journal PNAS on June 15, 2020, revealed that hummingbirds can distinguish a variety of non-spectral colors that the human eye cannot perceive or even imagine.
This is due to the presence of an additional color cone in the bird's eye that is not present in the human eye. While humans have three color cones in the retina that are sensitive to red, green, and blue light, birds have a fourth color cone that can detect ultraviolet rays.
Excellent color vision
The research team, led by biologist Mary Caswell Stoddard of Princeton University, USA, trained wild hummingbirds to conduct a series of experiments that revealed that birds see mixed colors such as UV + green, UV + red.
This excellent color vision plays a vital role in bird behavior including finding food, dazzling couples, escaping from predators, and navigating through a variety of terrain.
"People have color blindness, compared to birds and many other animals," Stoddard says.
It says, "Having a fourth conical type of color does not only extend the range of visible colors of birds to ultraviolet radiation, but it may also allow birds to perceive complex colors like UV + green and UV + red, but it was difficult to test that ".
Non-spectral colors
The Stoddard team was especially interested in "non-spectral" color combinations, from which humans only see violet, because it arises when the receptors of blue and red are stimulated.
While humans have one non-spectral color that is violet, it is believed that birds, with their additional conical cells, may be able to perceive up to five non-spectral colors, including purple, UV + red, UV + green, UV + yellow, UV + violet.
Stoddard and her colleagues designed a series of experiments to test whether hummingbirds can see these non-spectral colors.
They trained the wild broad-tailed hummingbirds (Selasphorus platycercus) to participate in color vision experiments, so the team prepared LED tubes programmed to display a number of colors, including non-spectral colors that humans could not see, and then these devices were placed next to Water feeders, some contain sugar water (which birds love) next to one color, while others contain plain water along with a different color.
Then the scientists switched the positions of these nutrients, and see if the birds can use the color index to know which nutrient it is.
The world with the eyes of hummingbirds
In a series of randomized trials over a three-year period (which included thousands of feeding sessions), the aim was to find out whether hummingbirds showed a preference for feeding in water plants displaying UV color combinations, which would support their vision.
The results show that animals can easily distinguish different types of non-spectral colors for a sweet treat.
"It was wonderful to see ... while UV + green and green rays seemed identical to us, hummingbirds were still choosing correctly the UV + green associated with water," says co-author and doctoral author Harold Easter of the University of British Columbia. Sugar".
"Our experiences have enabled us to peek at what the world looks like for the hummingbird," he added.
How does it look
The researchers explained that showing that birds can distinguish a variety of non-spectral colors provides a step forward in our understanding of the dimensions of bird vision.
However, there is still much that we do not know about the neuronal mechanisms that enable animal brains to perceive these colors, and much remains to be learned.
The researchers also point out that the deepest question is what do "non-spectral" colors look in the eyes of hummingbirds? Does UV + green appear to birds as a mixture of these colors or as a fresh new color?
We can't figure it out now, and thinking about this indisputable question about invisible colors is a pure human burden.
But in the end, what matters most to the bird is probably not the way the colors appear, but the way these colors work to provide information about food, couples, or predators.