The tiny birds’ ability to see colors outside the rainbow is “one of the most exciting things I’ve ever witnessed,” one scientist says.
– National Geographic – 16 June 2020
“A male hummingbird simply pausing on a perch can mesmerize us with his colorful, iridescent plumage. But it turns out we humans are likely missing the full effect—because hummingbirds see colors that humans cannot detect, a new study says.
Scientists have long known that birds probably have better color vision than humans do. Like most primates, humans are trichromatic—that is, our eyes have three types of color-sensitive receptors or cones: blue, green, and red. But birds have four color cones, meaning they are tetrachromatic.
With our three color cones, we can see the colors of the rainbow—red, orange, yellow, green, blue, indigo, and violet—the so-called spectral hues. We can also see one pure nonspectral (meaning, not in the rainbow) color, purple, because it stimulates our red and blue cones simultaneously.
Birds’ four color cones theoretically let them discriminate a broader range of colors, including the ultraviolet spectrum, which includes colors such as UV-green and UV-red. But so far, researchers have made few investigations into what birds can actually.
Then Mary Stoddard, a Princeton University evolutionary biologist, and colleagues carried out a series of field experiments with wild broad-tailed hummingbirds near the Rocky Mountain Biological Laboratory in Colorado. The remarkable results revealed the birds could discern spectral-colored feeders from feeders in nonspectral colors.
“Seeing them do this right in front of my eyes is one of the most exciting things I’ve ever witnessed,” says Stoddard, whose paper published this week in the journal Proceedings of National Academy of Sciences.
The study is a “big step forward,” offering the most thorough look at how birds distinguish color to date, says Trevor Price, an evolutionary biologist at the University of Chicago who wasn’t involved in the research.
“We’re really only beginning to scratch the surface in our understanding of color vision in animals,” he says.
For the study, Stoddard and her team set up several tube bird feeders outfitted with LED devices near the laboratory. They programmed LED devices near the feeders to turn a surface two different colors, depending upon whether the feeder held either a small amount of sugary water or plain water.
“It’s important to do the tests in the wild,” says Stoddard, “so that we can really understand these birds’ sensory experience of the world.”
The hummingbirds, which eat flower nectar, quickly learned to associate one color with a rewarding sweet sip, and the other color with unrewarding plain water.
Over three field seasons from 2016 to 2018, the scientists conducted 19 experiments and tallied some 6,000 hummingbird visits. By tracking the birds’ visits to the feeders, the scientists showed that broad-tailed hummingbirds consistently chose the feeder with the sweet taste, regardless of whether they had a nonspectral or spectral hue. (Learn what makes hummingbirds such excellent fliers.)
“Even when the colors looked the same to us—for example, when the birds had to choose from a feeder colored UV-green and one that was simply green—they could see the difference,” says Stoddard.
“It was an amazingly bold experimental approach,” says Karen Carleton, an evolutionary biologist at the University of Maryland, College Park, via email. The study shows that, “through hummingbird eyes, the world might look totally different to what we see.”
In living color
Color vision helps animals select their food and mates and avoid predators. Bees, for instance, can see ultraviolet bull’s-eye patterns in yellow flowers that directs them to the nectar. When we look at the same flower, we see only a yellow blossom.
To determine why hummingbirds see such a variety of colors, Stoddard and colleagues scientists analyzed existing data of various bird plumage color and plant colors. They found that hummingbirds would be able to see 30 percent of the birds’ plumage and 35 percent of the plant colors in nonspectral hues—colors that “humans cannot even imagine,” Stoddard says. This ability likely helps the small birds locate a diverse variety of plants and their nectar.
Stoddard and her team believe their results apply to all birds with tetrachromatic vision that are active during the day, as well as several fish, reptiles, and invertebrates. And this extra level of discernment might also have been a trait of dinosaurs, which are thought to have sported colorful feathers. (Read how jumping spiders can see more colors than you.)
Mammals evolved as nocturnal beings that did not need to see the rich hues of the daylight world, so most—like your pet dog and cat—are dichromatic, and have only blue and green cones. People evolved a third cone (red), possibly because early primates developed an appetite for ripening fruits.
“If we are ever going to understand the diversity of color in nature, we surely have to understand how species differ in their ability to perceive color,” adds Price. “This study shows the way.”