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First published online January 3, 2006
Journal of Experimental Biology 209, 380-390 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01988

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Iridescent plumage in satin bowerbirds: structure, mechanisms and nanostructural predictors of individual variation in colour

Stéphanie M. Doucet^1,*, Matthew D. Shawkey^1,, Geoffrey E. Hill¹ and Robert Montgomerie²

¹ Department of Biological Sciences, 331Funchess Hall, Auburn University, Auburn, AL 36849, USA
² Department of Biology, Queen's University, Kingston, ON, Canada K7L 3N6

View larger version (12K): [in a new window]   Fig. 1. Schematic representation of measurement geometries. Angles of incident (i) and reflected (r) light increase when moving away from a plane perpendicular to the reflective surface and are always equivalent. Reflectance measurements were taken at incident light angles of 0° (perpendicular to the feather surface) except where otherwise indicated.

View larger version (25K): [in a new window]   Fig. 2. Schematic representation of the four different thin-film models investigated in this study. Curved arrows represent beams of light. The models represent all possible combinations of layers and interfaces for the upper surface of the iridescent barbules of adult male satin bowerbirds.

View larger version (88K): [in a new window]   Fig. 3. Comparison of the feathers of adult (A,C,E,G) and juvenile (B,D,F,H) male satin bowerbirds. (A,B) Photographs of feathers; (C,D) scanning electron micrographs of barbs and barbules; (E–H) transmission electron micrographs of barbule cross-sections. In transmission electron micrographs, the dark ovals are melanin granules and the grey areas are keratin. Scale bars, 200 µm (C,D), 5 µm (E,F), 800 nm (G,H). Note that the scanning electron micrograph shown in C was taken at the proximal end of feather barbs to allow visualization of the twisting and flattening of barbules. The distal ends of feather barbs are much more densely covered with barbules.

View larger version (14K): [in a new window]   Fig. 4. Comparison of adult male satin bowerbird reflectance spectra, either measured using a spectrometer (solid lines) or predicted from thin-film optical models (broken and dotted lines). Predicted spectra are modeled based on measurements of barbule nanostructure from the individual whose measured spectrum is shown. Model 2 (dotted line) closely predicted measured spectra (solid line). By contrast, Models 1 (dashes and dots), 3 (short dashes), and 4 (long dashes) were poor predictors of measured spectra.

View larger version (23K): [in a new window]   Fig. 5. Graphical representation of the effect of changing various parameters on the predictive ability of thin-film Model 2. (A,B) The solid line shows Model 2 calculated from the variables as described in the text (extinction coefficient k of keratin=0.03, melanin=0.6; refractive index n of keratin=1.56, melanin=2.0). (A) The effects of increasing k of keratin to 0.06 (dotted line) and melanin to 1.0 (broken line). (B) The effects of increasing n of melanin to 2.2 (dotted line) and decreasing n of melanin to 1.8 (broken line).

View larger version (14K): [in a new window]   Fig. 6. Reflectance of adult male satin bowerbird barbules predicted by Model 2. The dotted line separates bird-visible wavelengths (300–700 nm) from near-infrared wavelengths (700–1400 nm). The bird-visible reflectance peak is a second-order harmonic of the fundamental peak in the near-infrared.

View larger version (13K): [in a new window]   Fig. 7. Relation between cortex thickness and hues predicted from thin-film optical theory using Model 2 (broken line) and hues measured with a reflectance spectrometer (solid circles) for the iridescent rump feathers of adult male satin bowerbirds.

View larger version (18K): [in a new window]   Fig. 8. Changes in brightness (diamonds) and saturation (circles) of adult male bowerbird feathers measured at different angles of incidence.

View larger version (14K): [in a new window]   Fig. 9. Relationship between hues predicted from thin-film optical theory (Model 2) and hues measured with a reflectance spectrometer for the iridescent rump feathers of adult male satin bowerbirds measured at six different angles of incidence (angles shown above symbols).