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Peacock feathers
In 1634, Sir Theodore de Mayerne, physician to Charles I, observed that the 'eyes' on the wings of the peacock butterfly "shine curiously like stars, and do cast about them sparks of the colour of the Rainbow; by these marks is it so known that it would be needless to describe the rest of the body though painted with a variety of colours."
Peacock speading its feathers. Every branch carries a series of bright colored spots, the actual color depending on the exact position of the spot on the branch and on the angle of the incident light.
Detail of a peacock feather. The iridescent plumage of the peacock originates in the fine side branches of the feathers.
In general, the diversity of bird feathers colors can be explained by mere two factors: pigments on one side and simple structures in the feathers which interfere with the incident light. Pigment particles are embedded into the newly grown feathers during the molting season. They absorb light of certain wavelength or disperse the reflected light and thus contribute to the color of the plumage. Colors due to pigments are explained in the section about organic pigments. Here we concentrate on colors due to interference.
What is a peacock feather up close?
Each feather consists of thousands of flat branches (as above in the detail). When like shines on the feather, we see thousands of glimmering colored spots, each caused by minuscule bowl-shaped indentations. Stronger magnification reveals microscopic lamellae (thin platelike layers) at the bottom of the indentations (see photo below). Just like in butterfly wings, the regular pattern of the lamellas leads to interference phenomena and thus to iridescent colors. This principle is also responsible for varied colors of pheasants, birds of paradise and humming birds.
Is it really just interference?
Yes. For example, the white color of this albino peacock is due to the missing black melanine pigment. The usual rich colors of the peacock are seen because black pigment which absorbs most of the incident light, allowing us to see only the interferece colors. In this peacock, the interference is still happening, but the effect is entirely washed out by the abundance of white light. In this albino, you can see that the "eyes" of the tail feathers are clear, not colored.
