The tropical forests of New Guinea, Indonesia and Australia are home to one of the most extravagant groups of birds, birds-of-paradise. Males of the 45 species display mesmerizing dances and plumage ornaments, with notorious complexity and diversity, to attract and mate with females. But any good show requires a good costume, so a group of researchers from the Natural History Museum of New York, USA, investigated how the birds-of-paradise get their bright colours.
Rene Martin and the team specifically looked for bio-fluorescent feathers. In addition to reflecting light, which we see as reds, greens and blues, fluorescent structures absorb shorter wavelength light and re-emit it at longer wavelengths, appearing vividly brighter. The wavelength of light determines its colour, so fluorescent objects absorb blue and UV light and re-emit it as yellow or green light. Many marine species, some arthropods, frogs and three other bird groups also have fluorescent body parts; might birds-of-paradise be members of the club too? Wearing blue-filter goggles to block reflections, the scientists shone blue flashlights on specimens of bird-of-paradise skin, complete with feathers, in a darkened room of the zoological collection, looking for glowing green and yellow patches. Impressively, they found fluorescent body parts in 37 of the 45 bird-of-paradise species: on white and yellow feathers, as well as legs, bill (outside and inside too) and the bare skin of some species. Based on the occurrence of fluorescence in the different species, the team reconstructed the evolutionary history of fluorescence in birds-of-paradise and found that it probably appeared in the ancestor of all birds-of-paradise species, although one has lost the ability to fluoresce.
The researchers subsequently measured the fluorescence spectrum and intensity of various feathers and found that the intensity varied strongly between species and body parts. In particular, the fluorescent feathers of females were markedly fainter than those of males. The team asked whether the males’ stronger fluorescence serves in the incredible displays that the birds-of-paradise put on to attract females, specifically as fluorescence in animals could arise by chance as a functionless by-product of another process. The researchers then checked whether birds are able to see the fluorescence in natural circumstances and it turns out that bird-of-paradise vision is tuned particularly to shades of yellow and green. The birds also have inbuilt filters in the eyes to help them distinguish close shades. But why have the birds expanded their colour pallet to include fluorescent shades?
Many birds-of-paradise perform their courtship displays in the dingy conditions beneath the forest canopy, where most of the light has been absorbed by the vegetation above. In these semi-dark environments, there is more UV and less red light in the spectrum. This has two consequences: there is enough UV and blue light to produce fluorescent colours, and the fluorescence is bright enough to be seen in the dimly lit forest. In addition, the birds’ fluorescent patches tend to be located near dull, black or super-black feathers which absorb all of the light. This provides a sharp contrast with the background, making the fluorescent colours pop even more. The team also noted that most of the strongly fluorescent feathers and skin are instrumental during male displays; females don't even possess the brightest fluorescent ornaments that males have. In addition, the team speculate that some females may use biofluorescence for camouflage, as they revealed dim fluorescence in some of their mottled belly feathers. This could help them blend in with the light coming down from the canopy above when seen from below. All in all, the scientists found strong indication that birds-of-paradise use fluorescence to make their ornaments stand out and their displays even more eye-catching in the poorly lit forest understory.
