Male birds of paradise have exceptionally black feathers and now researchers in the US have explained how the feathers manage to reflect tiny amounts of light. The team found that some feathers have complicated structures that create a scattering effect that results in almost zero reflectance of light under certain conditions – giving them a “super-black” appearance. The researchers think that this black plumage evolved to enhance the perceived brilliance of adjacent colour patches during courtship displays (see figure).
Birds of paradise are found in New Guinea and parts of eastern Australia. They are famous for the elaborate courting displays, plumage ornaments and dramatic colouration of the males. In many species, males have brightly coloured patches of feathers next to matte black plumage that appears much darker than the black colouration of other birds.
When researchers from Harvard University, the Smithsonian Institution, and Yale University shone light on museum specimens of five species of the bird of paradise they discovered that these black feathers have an extremely low directional reflectance – at normal incidence they only reflect back 0.05–0.31% of light. In contrast, black feathers from two other species of bird, used for comparison, had a directional reflectance of 3.2–4.7%. This extremely low directional reflectance extends over the entire visible range.
Scanning electron microscopy and high-resolution synchrotron tomography done by the team revealed that this is a result of the feathers microscopic structure. A typical feather has a central shaft with rows of barbs branching off. Rows of smaller barbules then spread out from the barbs. In most feathers this structure is flat, with everything laying in the same horizontal plane. But the super-black feathers have barbules that are covered in microscale spikes and they curve away from the horizontal plane.
The researchers explain that these vertically-tilted barbule arrays create deep, curved cavities that cause multiple scattering of light, resulting in more structural absorption of light than normal black feathers. Dakota McCoy, from Harvard University, told Physics World: “Because they have very little exposed horizontal surface area, light is multiply scattered within their vertically-projecting barbules.” These super-black feathers even retained their black appearance when coated with gold dust, whereas the normal black feathers appeared gold”.
The modified barbules are only present on the exposed overlapping tips of the feathers, while those towards the base of the feathers have a typical feather structure. Also, the black feathers from the back of one bird of paradise species, the superb bird-of-paradise, Lophorina superba, which are not used during display, had a typical barbule morphology and were more reflective than the super-black feathers. This supports the idea that the modified feathers have evolved for display purposes, the researchers say.
Evolved optical illusion
The super-black feathers appear darkest when viewed from straight ahead and during courtship displays males of many bird of paradise species move around to maintain a specific directional orientation between themselves and the female bird. The team believes that the plumage has evolved to enhance the perceived brilliance of adjacent colour patches during these displays and that the modified feathers combined with display behaviour ensure that female birds view the black plumage at its darkest. “It is an evolved optical illusion, which makes adjacent colours appear even brighter,” explains McCoy. “In effect, it is making our eyes and brains perceive the colour as being ‘in the shade’, as if there is almost no light illuminating it.”
Silvia Vignolini, at the University of Cambridge, says that it is important to note that directional reflectance is not the same as absorption, and that in her opinion “the recorded value of blackness reported in the paper is not that impressive”. She explains that while the feathers demonstrate extreme directional reflectance if viewed from the correct angle their total reflectivity is not that different to the other species examined. She adds that to be “very” black you need 100% absorption of light from all angles.
The figures in the paper show that while directional reflectance of light from the modified feathers is 0.05–0.31%, compared with 3.2–4.7% from the normal black feathers, their total reflectance considering all angles is around 3.5%, compared with 4.5–5.5% for the unmodified feathers.
McCoy says that the structure of the feathers could perhaps be used to design new absorptive surfaces for textile manufacture, optical instrumentation, or solar collection. She adds: “Feathers are very durable and are made of a cheap material – keratin – so it is possible that these feathers could be the template for a durable, scalable super-black surface.” Vignolini also sees practical applications, saying that the findings could help improve the anti-reflection properties of matte coatings.
The study is described in Nature Communications.