A landmark study revealed that hummingbirds perceive a much broader spectrum of colors than mammals thanks to a unique biological feature.
A male broad-tailed hummingbird flies in Colorado as part of an experiment on color vision.
A male hummingbird simply resting on a branch can fascinate human observers with its colorful, iridescent plumage. Now it turns out that we humans are probably missing out on the full effect of the play of colors – because hummingbirds see colors that humans can’t even recognize.
Scientists have known for some time that birds probably have better color perception than humans. Like most primates, humans are trichromatic, meaning that our eyes have three types of color-sensitive receptors, or cones: blue, green and red. Birds, on the other hand, have four cones and are therefore tetrachromatic.
With our three cones, we can see the colors of the rainbow, called spectral colors: red, orange, yellow, green, blue, indigo, and violet. In addition, we can also see a non-spectral color, purple, because it appeals to our red and blue cones at the same time.
Their four cones theoretically allow birds to perceive a wider range of colors, including the ultraviolet spectrum, which includes colors such as UV green and UV red. But until now, researchers have done little research on what birds can actually see.
Curtain up for Mary Stoddard. The Princeton University evolutionary biologist and her colleagues conducted a series of field experiments with wild hummingbirds near the Rocky Mountain Biological Laboratory in Colorado. The remarkable results showed that the birds could distinguish spectrally colored food from food in non-spectral colors.
"When I saw them do that right in front of my eyes – that was one of the most exciting things I’ve ever experienced," says Stoddard, whose study was published in the journal Proceedings of National Academy of Sciences.
The publication is a "major advance" and offers the most thorough look yet at how birds distinguish colors, judges Trevor Price. The evolutionary biologist at the University of Chicago was not involved in the research.
"We’re really just starting to scratch the surface in terms of our knowledge of animal color perception," he says.
Play of colors in the water
For the study, Stoddard and her team set up several water dispensers equipped with LED lights near the lab. They programmed the LED devices so that the surface of the liquids each took on one of two colors – depending on whether the food dispenser contained sugary water or plain water.
"It’s important to do the tests in the wild," says Stoddard, "so we can really understand how these birds perceive their world."
The hummingbirds, which feed on flower nectar, quickly learned to associate one color with a particularly tasty sweet sip and the other with regular water.
Gallery: the invisible glow of flowers
Over three seasons in the field from 2016 to 2018, the scientists conducted a total of 19 experiments and counted about 6.000 hummingbird visits to their feeders. By tracking which donor the birds sought out, the researchers were able to show that broad-tailed hummingbirds consistently chose the feeder with the sweet water – regardless of whether it had a nonspectral or spectral hue.
"Even when the colors looked identical to us – for example, when the birds had to choose between a feeder in UV green and one in normal green – they could see the difference," Stoddard says.
"It was an amazingly bold experimental approach," wrote evolutionary biologist Karen Carleton of the University of Maryland in College Park via email. He says the study shows that "the world through the eyes of a hummingbird could look very different from what we see".
Amazingly colorful world
Color perception helps birds choose their food and mates, as well as avoid predators. Bees, for example, can see an ultraviolet pattern in yellow flowers that directs them to the nectar like a target. When we look at the same flower, all we see is a yellow bloom.
To find out why hummingbirds see such a variety of colors, Stoddard and her colleagues analyzed existing data on the colors of different plumages and plants. They discovered that hummingbirds are able to see 30 percent of bird plumage and 35 percent of plant colors in nonspectral hues that "humans can’t even imagine," Stoddard says. This ability likely helps the tiny birds detect a wide variety of plants and their nectar.