Blackbirds, it turns out, aren’t actually all that black. Their feathers absorb most of the visible light that hits them, but still reflect between 3 and 5 percent of it. For really black plumage, you need to travel to Papua New Guinea and track down the birds-of-paradise.
Although these birds are best known for their gaudy, kaleidoscopic colors, some species also have profoundly black feathers. The feathers ruthlessly swallow light and, with it, all hints of edge or contour. They make body parts seem less like parts of an actual animal and more like gaping voids in reality. They’re blacker than black. None more black.
A typical bird feather has a central shaft called a rachis. Thin branches, or barbs, sprout from the rachis, and even thinner branches—barbules—sprout from the barbs. The whole arrangement is flat, with the rachis, barbs, and barbules all lying on the same plane. The super-black feathers of birds-of-paradise, meanwhile, look very different. Their barbules, instead of lying flat, curve upward. And instead of being smooth cylinders, they are studded in minuscule spikes. “It’s hard to describe,” says McCoy. “It’s like a little bottlebrush or a piece of coral.”
These unique structures excel at capturing light. When light hits a normal feather, it finds a series of horizontal surfaces, and can easily bounce off. But when light hits a super-black feather, it finds a tangled mess of mostly vertical surfaces. Instead of being reflected away, it bounces repeatedly between the barbules and their spikes. With each bounce, a little more of it gets absorbed. Light loses itself within the feathers.
McCoy and her colleagues, including Teresa Feo from the National Museum of Natural History, showed that this light-trapping nanotechnology can absorb up to 99.95 percent of incoming light. That’s between 10 and 100 times better than the feathers of most other black birds, like crows or blackbirds. It’s also only just short of the blackest materials that humans have designed. Vantablack, an eerily black substance produced by the British company Surrey Nanosystems, can absorb 99.965 percent of incoming light. It consists of a forest of vertical carbon nanotubes that are “grown” at more than 750 degrees Fahrenheit. The birds-of-paradise mass-produce similar forests, using only biological materials, at body temperature.
Vantablack is genuinely amazing: It’s so good at absorbing light that if you move a laser onto it, the red dot disappears. But McCoy has created a similar demonstration with her super-black feathers. In the image below, you can see two feathers, both of which have been sprinkled with gold dust. The left one is from the lesser melampitta—a bird of average blackness—and it looks as golden as its surroundings. The right one comes from a paradise riflebird—one of the 42 species of bird-of-paradise. Yes, it is covered in gold dust. And yes, it still looks black. The gold settles within the grooves of microscopic forest, and all of its glitter is lost.
This opens up several other questions, says Rafael Maia from Columbia University, who studies the evolution of bird colors. “Is this something unique to birds-of-paradise, or have other species evolved similar optical solutions?” he says. “If they have, do they use the same type of feather modifications?”
Many animals and plants use microscopic structures to produce exceptionally vivid colors with metallic sheens; this is called iridescence. Comparably fewer species use microscopic structures for the opposite purpose: to absorb colors entirely. These include a few butterflies and the Gaboon viper.
The viper—whose fangs, at two inches, are the longest of any snake—likely uses its super-black scales for camouflage, breaking up its outline so that the rest of its body better blends into the leaf litter of a rainforest. The birds-of-paradise, meanwhile, probably use their unfeasibly black blacks for the same thing that seems to motivate everything about them: sex.
“These likely evolved as an optical illusion, to make adjacent colors seem even brighter than they are,” says McCoy. “Animal eyes and brains are wired to control for the amount of ambient light. That’s why an apple looks red whether it is in the sun or the shade, even though the wavelength hitting our eyes is quite different in those scenarios. A super-black frame inhibits this ability, so nearby colors look like they are very bright—even glowing.”
The male birds use this illusion to great effect. The magnificent riflebird—that’s its adjective, not mine—splays out his super-black wings and flicks his head between them, showing off his electric blue throat. The superb bird-of-paradise—again, that is literally its name—spreads a cape of super-black feathers to highlight the electric blue patches on his cheeks and chest. He ends up looking like a spectral, wide-mouthed face. The six-plumed bird-of-paradise erects a super-black tutu and shimmies about to show off his kaleidoscopic throat bib.
The illusions work best when viewed straight on. From that angle, the little barbules and spikes are pointing straight at you, and they become better at trapping light. When viewed from the side, the super-blacks lose some of their blackness. That’s why the dancing males take such care to face the objects of their attention, bouncing around so their audience never gets a side view.
Super-black surfaces have plenty of uses for humans, too. They could camouflage military vehicles, help solar panels collect more light, or stop stray light from entering telescopes, improving the ability to spot faint stars. Vantablack can already do all of the above, but McCoy thinks the structure in super-black feathers might still be useful to engineers. “If these could be really cheaply 3-D printed, that would be amazing,” she says.
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