Birds are known for their visual prowess. Eagles are famously capable of seeing objects at a distance in vivid detail—they have roughly 20/5 vision compared to standard-issue human 20/20. But birds’ ocular superiority doesn’t stop there. Bird eyes also contain four types of color receptors, called cones, while humans have just three, which are sensitive to blue, green and red light, respectively.
This fourth type of cone possessed by birds is tuned to ultraviolet (UV) light, which is a part of the light spectrum that human eyes can’t see. Now, a new study of hummingbirds suggests these birds see the world in an astounding number of distinct colors that blend UV hues with the ones humans can also see, reports Virginia Morell for National Geographic.
The study results suggest hummingbirds—and perhaps all bird, reptile and fish species that possess the fourth type of UV-sensitive cones—experience a world awash in extra colors humans can’t imagine including UV-green, UV-red and UV-yellow. For birds, these additional colors don’t just paint a pretty picture, they are likely essential for finding food, picking mates and escaping predators, according to the paper published this week in the journal Proceedings of the National Academy of Sciences.
You might say the color purple is what led the researchers to conduct their experiments on hummingbird vision. For humans, purple is the clearest example of what’s known as a non-spectral color, which is a color created by combining wavelengths of light at different ends of the spectrum.
Inside the human eye, this means purple activates both our red cones, which are stimulated by long wavelength light and blue cones, which are stimulated by short wavelength light, according to a statement. By contrast, blended spectral colors, such as teal—blue plus green—or yellow—green plus red—mix things up with their neighbors of similar wavelengths on the electromagnetic spectrum.
Given that the spectral colors birds can perceive extend to the UV spectrum, evolutionary biologist Mary Caswell Stoddard of Princeton University wanted to see if that meant they could perceive and distinguish between non-spectral colors involving UV light. The problem, she says in the statement, is that this has been hard to test.
To investigate, the researchers conducted experiments with wild broad-tailed hummingbirds (Selasphorus platycercus) in the Rocky Mountains to determine whether they could perceive a difference between bird feeders marked by light tubes emitting spectral colors and those marked with non-spectral colors involving UV light, per National Geographic.
Stoddard’s team filled some of the feeders with a sugary mixture prized by hummingbirds and another with plain old water, reports Brook Hays for United Press International. "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 tells UPI.
Over the course of three years, 19 experiments and around 6,000 hummingbird visits, the experiments revealed the hummingbirds could perceive five non-spectral colors, reports Michael Le Page of New Scientist. Besides purple, the birds could also discern UV plus green, UV plus red, UV plus yellow and UV plus purple. This means the pint-sized fliers could pick out a nectar-filled feeder illuminated with UV plus green from a lineup including other feeders marked by pure ultraviolet or pure green light, per New Scientist.
Trevor Price, an evolutionary biologist at the University of Chicago who was not involved in the research, tells National Geographic that the results are a “big step forward” in our understanding of how birds can use colors to help navigate the world. He adds, “we’re really only beginning to scratch the surface in our understanding of color vision in animals.”
Karen Carleton, an evolutionary biologist at the University of Maryland who was also not involved in the new paper, tells National Geographic that the “bold experimental approach” of the study demonstrates that “through hummingbird eyes, the world might look totally different to what we see.”
What the experiments can’t do is tell us what those extra colors actually look like. “The more philosophical question of what these colors look like to birds is impossible for us to answer,” Stoddard tells New Scientist. “We have no idea what these colors really look like to birds.”
What the study made clear however was that the world birds live in is filled with these non-spectral UV shades. The researchers looked at 3,315 feather and plant colors and found that between 30 to 35 percent of them feature non-spectral colors, per National Geographic. This means that for hummingbirds, plumage or flowers which appear indistinguishable to us showcase obvious differences that help them pick the perfect made or zero in on a tasty meal.
Stoddard says in the statement that this non-spectral vision likely isn’t limited to hummingbirds. The fourth type of UV-sensitive cone is present in all birds, many fish, reptiles and probably dinosaurs, which Stoddard says may mean “the ability to perceive many non-spectral colors is not just a feat of hummingbirds but a widespread feature of animal color vision.”