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In puffin courtship, making a good impression is all about the beak. When one Atlantic Puffin approaches another, the two birds bow their heads and rub their candy-corn bills together while rapidly shaking their faces back and forth. Sometimes, a crowd of bobbing puffins will form around the pair, egging them on.

Now, scientists have discovered that there is more to this display of affection than meets the human eye. That’s because if you shine ultraviolet light on a puffin in breeding plumage, streaks on its beak glow a bright cyan-green color. The discovery could help us see puffins as they see themselves—and force us to reexamine our assumptions about how birds and other animals experience the world and each other. 

For Jamie Dunning, lead author of on puffin beak luminescence published in the journal Bird Study, shining an ultraviolent light on a puffin beak seemed like a natural thing to do. As he knew, all birds live in a world of color humans can barely imagine. While our retinas have three kinds of photoreceptors, called cones, to pick up red, green, and blue wavelengths of light, bird retinas have four, letting them see red, green, blue, and ultraviolet light.

Many bird species have UV adornments that are invisible to us. Scientists are just beginning to understand how birds make use of this visual superpower, and many suspect it plays a role in mating rituals. For example, songbirds like Blue Jays or Wood Thrushes have ultraviolet feathers that indicate gender. And female Budgerigars, little green parakeets native to Australia, prefer males with elaborate, UV-reactive plumage.

Though it wasn’t the focus of his research, Dunning was always curious about how other species use those extra wavelengths of light. The Atlantic Puffin and its flashy bill stood out as a possible contender. “That big, beautiful, colorful ornament seemed like a good place to start,” he says.

So in 2018, on a slow day in his former lab at the University of Nottingham in the United Kingdom, Dunning, now an environmental consultant, put a puffin specimen (which had died recently of natural causes) under a black light. Immediately, the bill seemed to glow.  

He shared the results on , and then circulated a write-up among fellow scientists. Tony Diamond, a biologist in Canada, reached out to Dunning and said he had observed the same phenomenon back in 2010. Dunning invited him to collaborate, and the two put together a team to take a closer look at the phenomenon—this time, on the bills of living puffins. 

First, the researchers had to guarantee the puffins’ safety. The scientists weren’t sure if exposing the birds’ eyes to ultraviolet light would be harmful and decided not to risk finding out. Dunning enlisted the help of designers at Goldsmiths, University of London to create a prototype of auk to protect the bird’s eyes from the light. Made mostly of lightweight foam and waterproof neoprene, the opaque design perches on a puffin’s beak to shield its eyes. 

The designers sent a of the sunglasses to researchers in the field, who worked with the wild birds as quickly as possible to minimize contact, says Dunning. Ultimately, it was Daniel Hanley, a behavioral ecologist at New York’s Long Island University Post and study co-author, who managed to measure the amount and intensity of light emitted by the beaks of three live puffins on Petit Manin Island off the coast of Maine. Each beak luminesced, too.

“This is an exciting study and emphasizes that there’s more to these birds than what we think we know,” Christy Wails, a PhD candidate studying avian ecology at Northern Illinois University, wrote in an email to ���Ϲ���. What’s more, puffins are not the only auks with rave-ready beaks: A couple years ago, Wails and her colleagues that Crested Auklets, a puffin relative, also have luminescent bills.

All current theories about why auks have luminescent beaks revolve around breeding. For one thing, that distinct puffin beak is seasonal. Both male and female puffins grow colorful ornamental plates, UV-activated streaks and all, for the summer months and discard them for a simpler, less colorful bill when the birds abscond to the high seas for winter. The birds also make good use of their beaks during mating rituals—a behavior called, appropriately, billing—suggesting that the ornamentation may have evolved as a result of sexual selection. Crested Auklets, Wails notes, behave in a similar way and also have disposable ornamental bill plates.

“My money is on sexual signaling,” says Dunning, who thinks the luminescent marks could play a role in either mate attraction or defending nesting territory from interlopers. Disappointingly, it is unlikely the bill functions as a full-fledged flashlight, he says.

We may only be at the beginning of understanding luminescence in birds, but as far as puffins are concerned, we at least know that their mating ritual is more than cute. It’s psychedelic. 

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