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Abstract
Iridescence is produced by coherent scattering of light waves from alternating layers
of materials of different refractive indices. In birds, iridescent colours are produced
by feather barbules when light is scattered from alternating layers of keratin, melanin
and air. The structure and organization of these layers, and hence the appearance
of bird species with different types of plumage iridescence, varies extensively. One
principal distinction between different types of iridescent colours is whether they
are produced by a single pair of layers or by multiple pairs of layers. Multi-layer
iridescence, such as that displayed by hummingbirds, has been relatively well characterized,
but single-layer iridescence has only recently been modeled successfully. Here we
use electron microscopy, spectrometry and thin-film optical modeling to investigate
the glossy, ultraviolet-blue iridescent plumage colouration of adult male satin bowerbirds
Ptilonorhynchus violaceus minor. The flattened barbules of adult males are composed
of a superficial keratin layer overlying a melanin layer that is several granules
thick. A thin-film model based on the thickness of the keratin layer and its two associated
interfaces (air/keratin and keratin/melanin) generates predicted reflectance spectra
that closely match measured spectra. In addition, hues predicted from this model are
positively correlated with measured hues. As predicted from our thin-film model, measured
hues shifted to shorter wavelengths at increasing angles of incidence and reflectance.
Moreover, we found that individual variation in barbule nanostructure can predict
measured variation in both hue and UV-chroma. Thus, we have characterized the microstructure
of satin bowerbird barbules, uncovered the mechanisms responsible for producing ultraviolet
iridescence in these barbules, and provided the first evidence of a nanostructural
basis for individual variation in iridescent plumage colour.