Bufflehead (Bucephala albeola) Male and female. Photo Brendan Tidd.

Photographer Brendan Tidd caught this Bufflehead pair in just the right light to bring out the multicolored iridescence in the male’s headfeathers. The female dresses more conservatively, making do with a plain white splash on her cheek.

The Bufflehead male’s iridescence shows at least three colors: green, blue, and red, plus their mixtures or overlaps. Iridescence is widespread in nature. It is not the product of pigments, but of microstructures. The Wikipedia writers explain:

In biological (and biomimetic) uses, colours produced other than with pigments or dyes are called structural coloration. Microstructures, often multilayered, are used to produce bright but sometimes non-iridescent colours: quite elaborate arrangements are needed to avoid reflecting different colours in different directions.[12] Structural coloration has been understood in general terms since Robert Hooke‘s 1665 book Micrographia, where Hooke correctly noted that since the iridescence of a peacock‘s feather was lost when it was plunged into water, but reappeared when it was returned to the air, pigments could not be responsible.[13][14] It was later found that iridescence in the peacock is due to a complex photonic crystal.[15]

Undoubtedly the same photonic crystals are present in the Bufflehead’s head feathers. Drilling down further:

photonic crystal is a periodic optical nanostructure that affects the motion of photons in much the same way that ionic lattices affect electrons in solids. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, in different forms, promise to be useful in a range of applications.

In 1887 the English physicist Lord Rayleigh experimented with periodic multi-layer dielectric stacks, showing they had a photonic band-gap in one dimension. Research interest grew with work in 1987 by Eli Yablonovitch and Sajeev John on periodic optical structures with more than one dimension—now called photonic crystals. …

Photonic crystals are composed of periodic dielectric, metallo-dielectric—or even superconductor microstructures or nanostructures that affect electromagnetic wave propagation in the same way that the periodic potential in a semiconductor crystal affects electrons by defining allowed and forbidden electronic energy bands. Photonic crystals contain regularly repeating regions of high and low dielectric constant. Photons (behaving as waves) either propagate through this structure or not, depending on their wavelength. Wavelengths that propagate are called modes, and groups of allowed modes form bands. Disallowed bands of wavelengths are called photonic band gaps. This gives rise to distinct optical phenomena, such as inhibition of spontaneous emission,[4] high-reflecting omni-directional mirrors, and low-loss-waveguiding. Intuitively, the bandgap of photonic crystals can be understood to arise from the destructive interference of multiple reflections of light propagating in the crystal at the interfaces of the high- and low- dielectric constant regions, akin to the bandgaps of electrons in solids.

The periodicity of the photonic crystal structure must be around half the wavelength of the electromagnetic waves to be diffracted. This is ~350 nm (blue) to ~650 nm (red) for photonic crystals that operate in the visible part of the spectrum—or even less, depending on average index of refraction. The repeating regions of high and low dielectric constant must, therefore, be fabricated at this scale, which is difficult.

While manufacturing photonic crystals in the laboratory presents many challenges, the Bufflehead male does it effortlessly just by being hatched.

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