White-throated Kingfisher: Non-iridescent colours

posted in: Morphology-Develop. | 0

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bklim photographed an adult White-throated Kingfisher (Halcyon smyrnensis) showing its brilliant colourful plumage – dark chestnut, blue and white. In addition, it has a red bill, dark brown iris, red orbital skin and legs. The female may have a slightly paler head and belly while the juvenile’s plumage is slightly duller. Whatever the sex or age, the bird is a spectacular specimen, guaranteed to impress anyone.

There is a popular misconception that the brilliance of the kingfishers’ colours is dependent on the angle of light, a result of iridescence. But iridescence does not come into play here, nor are the colours a direct result of the pigments.

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What are responsible are the microscopic structures of the feather.

The mature feather covering is made up of a hard protein sheath of keratin. Just below this sheath is a layer of keratin cells filled with tiny pockets of air. As white light strikes the feather, the short wavelengths are scattered by the air pockets. As shades of blue (blue, indigo and violet) have the shortest wavelengths, they are scattered the most and in all direction. Thus we see the blue from any angle.

Just below the layer of cells containing the light scattering air pockets are melanin, pigments that absorb most of the longer wavelengths of light. This creates a dark background, thus intensifying the blue we see.

Other non-iridescent colours besides blue are also produced structurally. When the light-scattering air pockets are a bit bigger (bigger than the wavelength of blue light), the result is green (since blue is no longer scattered, and green wavelengths are now scattered the most), as in some parrots.

With even larger air pockets, no wavelengths are scattered, but all are reflected, producing white light and thus plumage that we perceive as white; white does not exist as a pigment in birds.

All images by bklim.

Reference:
Clark, G. A. Jr. (2004). [‘Form and function: The external bird.’]. Pp. 3.1-3.70 in Podulka, S., Rohrbaugh, R.W. Jr & Bonney, R. (eds.) Handbook of bird biology. Ithaca, NY: The Cornell Lab of Ornithology.

This post is a cooperative effort between www.naturepixels.org and BESG to bring the study of bird behaviour through photography to a wider audience.

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