Birds-of-paradise are among the most resplendent creatures on Earth, with long, elaborate feathers in eye-popping shades of yellow, blue, and red. Naturalists have admired them for centuries. Yet it turns out they didnt know the half of it.As if those vibrant colors werent enough, a recent study found that the birds native to Australia, Indonesia, and New Guinea are also biofluorescent. Their skin and plumage absorb light at high-energy wavelengths, then re-emit it at low-energy wavelengths visible to other birds as a bright yellow-green. Basically, they harness the suns rays to make their already impressive displays even more dazzling.Biofluorescence by itself isnt big news; it shows up in plenty of animals, including humans (our teeth fluoresce under UV light). But almost always it either has no discernible function, or its used for camouflage. This, says lead author Rene Martin, a biologist at the University of Nebraska-Lincoln, is one of the few cases where were seeing biofluorescence is very likely being used as a signal. Thats where it tends to be rare.Evolution for DesireThe birds-of-paradise evolved optimal brilliance to communicate something, and that something is their own attractiveness: Like so many of their striking features, biofluorescence seems to help them entice mates. The females are very choosy, Martin says, and the males tend to be very flashy. Biofluorescence is just a small component of that.Martin and her colleagues tested all 45 species, using specimens from the American Museum of Natural History, and found biofluorescence in each of the 37 core birds-of-paradise. This suggests that their common ancestor was also biofluorescent, and for some reason the other eight species lost that trait over time. In those that retained biofluorescence, the trait has diversified wildly under the auspices of sexual selection. This is the lesser-known force that, along with natural selection, drives evolution. Whereas natural selection favors the fit, sexual selection favors the beautiful: The most desirable individuals are most likely to pass on their genes. Female choosiness can result in a positive feedback loop where exaggerated features grow more ornate from generation to generation, even to the point of conflicting with natural selection a male bird-of-paradise is conspicuous not only to mates, but also to predators. Nevertheless, sexual preference is a powerful sculptor. And birds have terrific eyesight, with visual acuity and color discrimination to rival any animal, so its no surprise these creatures evolved such a dramatic appearance. The Black Hole EffectBeyond biofluorescence, birds-of-paradise have another trick to enhance their look, and to stand out amid a noisy forest background. A 2018 study found that the birds colors arent just brighter, the surrounding areas are also darker theyve evolved super black feathers, which absorb more light than the feathers of other birds. This black hole effect creates an intense, irresistible contrast.Some species fan these plumes around their face to create a dark halo, then open their biofluorescent mouth for potential mates to appraise. In other species the females observe from above, so males spread their feathers out skirt-like to isolate a spark of biofluorescence on the crown of their head. They even choose their display sites carefully to exploit whatever sunlight is available to them. As Martin put it, they're playing with that light source to enhance their flashiness.Searching for a PurposeGiven that biofluorescence is often a mere byproduct of the structure of animal tissues, you cant just assume it serves some tangible purpose. To be sure that it does in birds-of-paradise, Martins team followed an established framework with five criteria. They proved four of them. The pigments are well-suited to absorb light in the birds natural habitat, theyre viewed against a contrasting background, theyre located in the relevant bodily regions, and the birds eyes are particularly sensitive to the re-emitted wavelengths.But theres one thing the researchers havent yet been able to test: Whether biofluorescence changes the behavior of females who behold it. Thats a tough one; the team would need to compare mating success in males with and without biofluorescence, so for the control group theyd need a way to turn off the UV light that excites that biofluorescence.We cant turn off the sun, of course, but it might be possible to run such an experiment with captive birds under artificial light. And the right doors seem to be opening already shortly after her paper was published, Martin received an email from a zookeeper who offered to collaborate. The woman didnt seem surprised to learn that birds-of-paradise were biofluorescent. Every time I see their feathers, Martin recalls her saying, I feel like there must be more.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Royal Society Open Science. Does biofluorescence enhance visual signals in birds-of-paradise?The Royal Society. Fluorescence as a means of colour signal enhancementCody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor's degree in journalism and media production from Washington State University.