Many male birds-of-paradise are noted for their bright colors and iridescent feathers, but several species of these tropical marvels also sport superblack plumage. Now researchers have teased out the structural secrets behind these light-absorbing feathers, which rival even the deep velvety blackness of some man-made materials.
Feathers, like most opaque objects, typically get their color one of two ways. The wavelengths of light that an object reflects depend on pigments or surface coatings (think: melanin in skin and chlorophyll in leaves, for example) or on tiny structures on or in its surface (think: iridescent butterflies and beetles). But the superblack feathers that some male birds-of-paradise use in their mating displays are pretty much the opposite of iridescent, says study co-author Dakota McCoy, an evolutionary biologist at Harvard University. Her team’s lab tests show the males’ superblack plumage absorbs up to 99.95 percent of the incident visible light. The findings were reported this week in Nature Communications.
A closer look at the feathers exposes their light-trapping trick: The tiny structures, or barbules, near the tip of these display feathers have a frayed appearance. By contrast, the barbules on flight feathers typically have small, Velcro-like hooks that help them latch onto their neighbors to form a solid yet flexible aerodynamic surface. When light strikes this forest of barbules, which is tilted about 30 degrees toward the outer tip of the feather, it gets reflected into deep cavities between the tiny structures rather than outward, McCoy says. The branched barbules seen on these feathers are wholly unlike the tiny structures that lend the scales on some types of snakes and butterflies their superblack color, the researchers note.
The Harvard team’s findings reveal a new type of microstructure for feathers, says Matthew Shawkey, an evolutionary biologist at Ghent University in Belgium, who was not involved in the work. “This structure enhances the blackness produced by the feathers’ pigments,” he adds.
Typical black feathers sported by birds-of-paradise—including some found elsewhere on one of the species the team analyzed—reflect between 10 and 100 times more light than the superblack feathers do, McCoy says. Further tests confirmed the feathers’ light-absorbing ability stems from their microstructure, she notes. The feathers remained velvety black even after they were coated with a thin sheen of gold whereas normal black feathers, as might be expected, appeared golden, she adds.
So why might the superblack feathers on male birds-of-paradise have evolved? It’s all about the males appealing to potential mates, McCoy and her colleagues propose. In all of the species the researchers examined in which males sport these velvety plumes, the superblack feathers are always immediately adjacent to bright, iridescent ones. During mating displays the males hold their velvety plumes such that the feathers appear their darkest from the females’ point of view, McCoy notes.
“This is definitely not about camouflage,” Shawkey says. “This superblack plumage is enhancing the contrast with those bright-colored feathers nearby.”