Apologies for the length of this piece … it become much more technical than anticipated, but I hope you will all find it interesting.
Structural Colouration Margie Tiller mentioned recently that Joan Paton told club members many years ago that the blue colour of birds is not from actual pigments. I thought I’d investigate a little further. Blue and iridescent colours in birds are never produced by pigments. They are "structural colours”. English scientists Robert Hooke and Isaac Newton first observed (from a scientific viewpoint) this phenomenon in living creatures. In Hooke’s 1665 book, Micrographia, he said of the peacock, “… each Feather in the tail sends out multitudes of Lateral branches, … so each of those threads in the Microscope appears a large long body, consisting of a multitude of bright reflecting parts. … their upper sides seem to me to consist of a multitude of thin plated bodies, which are exceeding thin, and lie very close together, and thereby, like mother of Pearl shells, do not only reflect a very brisk light, but tinge that light in a most curious manner; and by means of various positions, in respect of the light, they reflect back now one colour, and then another, and those most vividly.” A century later, Thomas Young (1773 – 1829) explained the principles behind structural colouration. Young was a British polymath who made notable contributions to the fields of vision and light, amongst other scientific discoveries. Structural coloration is the production of colour by microscopically structured surfaces fine enough to interfere with visible light. Peacock tail feathers are pigmented brown, but their microscopic structure makes them also reflect blue, turquoise, and green light, and they are often iridescent. It made me wonder about the real pigment colours of other birds we see as iridescent blue, green, purple etc. Iridescence occurs when the colour changes with the viewing angle and orientation. Structural colour, however, is no simple matter. In bird feathers, interference is created by a range of photonic mechanisms. The vivid colouration is caused by interference effects that reflect or scatter light, rather than by pigments. Colours are produced when a material is scored with fine parallel lines, formed of one or more parallel thin layers, or otherwise composed of microstructures on the scale of the colour's wavelength. The detailed structure of the feather’s barbules reflects some wavelengths and absorbs others, and the reflected wavelength changes with the angle of reflection. The structural colour is registered by the eye in response to the reflected wavelengths and changes with the angle formed by the light, the reflecting surface, and the eye. The benefits of structural colouration for birds include for camouflage, avoiding predation, signal communication and sex choice.
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