Feathers of the common kingfisher create colorful feathers due to pigment granules, spongy nanostructures, and thin films.
Introduction
Bird feathers come in literally all colors of the rainbow––and even some we can’t see. Some are produced by pigments, chemicals within the feathers that absorb or reflect different wavelengths of light differently. Others are produced by minute physical structures that bend or reflect select portions of the light spectrum. Variations in those physical structures can scatter light rays in slightly different ways, allowing the creation of bright, iridescent colors that can even change when viewed from different angles.
In the case of the common kingfisher, it’s both of the above. This eye-catching bird, which lives in Europe, Asia, and Africa, has three different colors: orange on the breast, cyan (greenish-blue) on its back, and blue on its tail. It uses both chemical and structural coloration to deck itself in attractive plumage. (No one knows exactly how the bird benefits, though it may have something to do with attracting mates or competing for resources.)
The Strategy
The feathers of common kingfishers, like those of other birds, are made up of a main stem, called a rachis, from which individual barbs branch off. However, kingfishers add a twist by having two distinct types of barbs. Both types have a thin outer layer called a cortex. But inside, they differ, and these differences produce different colors by different methods.
One type of barb, found on the bird’s chest, is made up of hollow cells with tiny granules of inside. The granules selectively absorb short-wavelength blue light, giving the feathers an orange hue. The barb cells are also irregularly spaced, which randomizes the direction of incoming light and scatters the long-wavelength red light.
The bird’s back and tail feature a second type of barb. This type is made up of cells that are spongy with a small bubble inside. The spongy tissue reflects short-wavelength blue light in many directions, producing bluish iridescence. The sponginess differs from the back to the tail reflecting different wavelengths of light and creating the appearance of cyan and blue, respectively. Beneath the spongy cells are other cells that use a pigment to absorb light that bounces in their direction, helping the blue colors to “pop” by removing other “competing” wavelengths from the picture.
On top of all of this, the cortex of the blue feathers varies minutely in thickness, and serves as a thin film that reflects light across the entire visible light spectrum. This adds additional shimmer to the overall look.
The spongy tissue reflects short-wavelength blue light in many directions, producing bluish iridescence.
The Potential
Color is one of the fundamental tools humans use to communicate, signal, and decorate. Apparel designers, paint manufacturers, and others in the business of color could apply the kingfishers’ strategy of mixing chemical and structural coloration to produce a range of effects with aesthetic as well as very practical applications.
