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Scientists have known for a few decades that birds can see colors that humans can’t. Our retina’s three cones—red, blue, and green—limit the range of color we can visually detect. But most birds have a fourth cone type that picks up ultraviolet light, which allows them to see a fourth dimension of color combinations. Just as mixing red and blue creates purple, birds might detect ultraviolet blended with red or green as a unique shade. That raised a question for , an evolutionary biologist and ornithologist at Yale University: Were birds seeing other birds in a completely different light?  

About 10 years ago, Prum, who studies the evolution of beauty in birds for decades, first looked at the colors displayed across 111 bird species, from penguins to parrots. Surprisingly, he found that, as a whole, they only produced colors comprising between one-fourth and one-third of the full breadth they’re capable of seeing. “Not all birds can make all colors,” he says. “They’re banging up against the limit of biochemistry and physics.”

Birds create colors in a number of ways. Many make melanin, a molecule that lends a raven its inky black or a canary its pale yellow. Others, such as the Northern Cardinal, ingest pigments through their diet, like the hundreds of types of plant-photosynthesized carotenoids that create reds, oranges, yellows, and pinks. Some birds owe much of their color to their feathers’ physical structure. Hummingbirds, Mallards, peacocks, and starlings get their iridescence from crystalline stacks of lozenge-shaped proteins in their feathers’ smallest filaments, which bend color depending on the angle of observation. A spongy, air-bubble-filled keratin feather structure results in a single, vivid blue in Blue Jays, for example. 

In earlier research, Prum and his colleagues noticed hummingbirds were, in particular, loaded with colors—especially structural hues. He suspected the avian family might be worth a closer lock. In a , he and his colleagues discovered that hummingbirds, with their vibrant greens, electric blues, and royal purples, are more colorful than humans had ever imagined, beyond what people are physically capable of seeing. In fact, their super-saturated plumages are remarkable even within the avian realm, out-huing all other bird species—combined. “We knew no matter what we did, if we studied hummingbird color, it would be interesting,” Prum said. “We didn’t know the answer would be so impressive.”

For the study, Prum and Gabriela Venable, the study’s lead author and a current graduate student at Duke University, used a pencil-size fiber optic wand to illuminate 1,600 patches of  feathers on the crowns, backs, tails, wings, bellies, and throats of preserved male specimens of 114 hummingbird species. Then they measured the wavelength of light reflected back, which is what animal eyes and brains process as a color. Their catalog of hues, they found, increased the total number of known plumage colors by more than half.

It’s not that other birds are not capable of being so vibrant, Prum explains. But because male hummingbirds mate with more than one female, they have especially strong reasons to maximize their potential. They rely on elaborate courtship displays to attract females, who then do the work of building nests and raising chicks. (In other words, female hummingbirds aren’t falling for their mates’ brains or work ethic.) The researchers further found that hummingbird crowns and throat patches are particularly colorful and can flash brilliantly toward observers, supporting the idea that this vibrancy could be for a female’s viewing pleasure. These colors aren’t just useful in mating—females and males alike use their bright plumage during antagonistic interactions to defend food sources.

Prum plans to continue studying the evolutionary role of structural colors in plumage, perhaps zooming back out to compare across bird families. But, as he’s argued for years, he still believes that in addition to being practical, the splendor of ornamentation itself can help us understand why birds evolved such kaleidoscopic palettes. “It’s like high fashion,” he says. “Birds are beautiful because they are beautiful to themselves.”

This story originally ran in the Winter 2022 issue as “Bird’s Eye View.” To receive our print magazine, become a member by .