Macaw´s blue feathers
Recently I got some blue feathers of a macaw (*1). Looking at them from different angles, I started to wonder about the iridescent blue color. From some angles, the feathers appeared more brown-grey rather than blue. So, I became curious and what I learned is that there is a difference between ‘pigmented color’ and ‘structural color’. There is no bird who could make blue from pigments, the blue depends on the light and the angle of incidence. It is the result of a selective reflection of the blue light by the structure of the surface of the feather. In nature there are lots of other examples of structural coloration in living creatures, to name a few examples there are the tail feathers of a peacock, the blue morpho butterflies, ‘green scarabs’(*2) which all obtain their colors by microscopically structured surfaces.
*1 The scarlet macaw's name is Pepe. He is around 30 years old and living with a good friend of ours in Ecuador. [scarlet macaw in spanish - Guacamayo Escarlata, Ara macao]
*2 Cotinis mutabilis
how does this work exactly in the case of the blue feathers?
"When visible light encounters particles with the same diameter or larger than its component wavelengths, those specific light photons are reflected. Such reflected light photons are collected and seen by the observer's eye, thereby imparting color to the perceived image. Because blue light has a very short wavelength, it is selectively reflected more easily than other colors of light with longer wavelengths [...]
The differences between structural and pigment colors can be demonstrated using several simple experiments. Because blue color is entirely dependent upon the reflective structure of the feather, it turns dark when ground up into a powder. However, a red or yellow feather retain their original color when subjected to the same treatment because pigments are not damaged when the feather structure is ruined. Pigments can also be removed from the feather without damaging its structure. When a red or yellow feather is placed into an appropriate solvent, the pigment granules will dissolve into the solvent, leaving behind a colorless feather. Blue feathers can also lose their blue coloring when placed into a liquid with a particular optical density, such as balsam, that fills the air cavities in the feather structure, thereby preventing reflection of blue light. Thus, such a feather appears dark when it is wet, but its lovely blue color returns after it has dried" (Bennu, D. (2000). Why are Bluebirds Blue? - The Physics of Structural Colors in Bird Feathers [Schemochromes]. International Turaco Society. 1st March 2000. Available at http://www.turacos.org/feathers.htm).
"For decades, scientists have known how birds with yellow or red feathers usually get their color: It comes from pigments in foods the birds eat. Flamingoes, for instance, extract pink pigments from algae and crustaceans they filter out of the water. The challenge has been to figure out exactly how blue birds get their color. It can’t be their diet: blue pigments, like those in blueberries, are destroyed when birds digest them. Scientists theorized that birds look blue for the same reason the sky looks blue: Red and yellow wavelengths pass through the atmosphere, but shorter blue wavelengths bounce off of particles and scatter, emitting a blue glow in every direction.
Richard Prum, an ornithologist at Yale, [...] discovered that as a blue feather grows, something amazing happens. Inside each cell, stringy keratin molecules separate from water, like oil from vinegar. When the cell dies, the water dries away and is replaced by air, leaving a structure of keratin protein interspersed with air pockets, like a sponge or a box of spaghetti. When white light strikes a blue feather, the keratin pattern causes red and yellow wavelengths to cancel each other out, while blue wavelengths of light reinforce and amplify one another and reflect back to the beholder’s eye. The result: blue, an example of what scientists call a structural color (as opposed to a pigmented color) because it’s generated by light interacting with a feather’s 3-D arrangement. And different shapes and sizes of these air pockets and keratin make different shades of blue" (Fields, H. (2012). 'Why Are Some Feathers Blue?'. Smithsonian Magazine. March 2012. Available at smithsonianmag.com/science-nature/why-are-some-feathers-blue-100492890/).
