The anatomical basis of sexual dichromatism in non-iridescent ultraviolet-blue structural coloration of feathers

Abstract

Despite extensive research on the evolution of avian dichromatism, the anatomical bases for differences between the sexes in species with structurally coloured plumage remain largely unknown. Using full‐spectrum spectrometry and transmission electron microscopy, we compared the colour and morphology of rump feathers of male and female eastern bluebirds (Sialia sialis). The ultraviolet (UV)‐blue feather colour in this species is caused by coherent scattering of light within the medullary ‘spongy layer’ of feather barbs. This spongy layer lies beneath a keratin cortex and on top of a layer of melanin granules that surround a hollow central vacuole. Irregularly shaped electron‐dense regions are present within the cortex. Male and female S. sialis differed substantially in their plumage colour and feather structure. A backwards logistic regression predicted sex with 100% accuracy using the colour variables brightness, UV‐violet (UV‐V) chroma and spectral saturation. A second backwards logistical regression predicted sex with 100% accuracy using relative cortex area and size of air spaces. Thus, S. sialis are dimorphic both in colour and in the structures causing this colour. Multiple regression analyses using data pooled from both sexes indicated that multiple features of feather barb structure contributed to colour variation in complex ways. Brightness was negatively related to the relative surface area of cortex in barb cross‐sections. Hue was positively related and UV‐V chroma was negatively related to the distance between scattering elements (i.e. keratin rods and air spaces) in the spongy layer. In contrast, hue was negatively related and UV‐V chroma was positively related to the thickness of the spongy layer. UV‐V chroma was also negatively related to the relative area of electron‐dense regions in the cortex. Spectral saturation was negatively related to the distance between scatterers and the standard error of the size of air spaces. These results suggest that the dimensions of spongy‐layer elements are critical to colour production, but that UV‐blue coloration can also be modified by the cortex and the thickness of the spongy layer. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 259–271.

The effect of rearing environment on blue structural coloration of eastern bluebirds (Sialia sialis)

“…The UV-blue feathers of eastern bluebirds are composed of a spongy medullary layer of feather barbs lying beneath a keratin cortex and above a layer of melanin granules surrounding large central vacuoles (Shawkey et al 2003). Brightness is related to the thickness of the keratin cortex that surrounds the medullary layer, such that birds with brighter plumage have feather barbs with thinner cortex (Shawkey et al 2005). Measures of spectral shape (UV chroma and hue), however, appear to be determined more by the precision of structural elements (keratin and air spaces) within the spongy layer (Shawkey et al 2003(Shawkey et al , 2005.…”

“…It is still unclear, however, whether noniridescent structural coloration can serve as a condition-dependent signal of individual quality. A few studies have shown that environmental perturbations can reduce structural color display (McGraw et al 2002;Johnsen et al 2003;Hill et al 2005, Jacot andPeters et al 2007) and that the precision of the arrangement of tissues at a nanostructural scale affects coloration (Shawkey et al 2003(Shawkey et al , 2005, but Prum (2006) argued that the mechanisms by which such structural coloration is produced makes condition dependency unlikely.…”

Structure and optical function of amorphous photonic nanostructures from avian feather barbs: a comparative small angle X-ray scattering (SAXS) analysis of 230 bird species

“…Fourier analyses of TEM images were sufficiently accurate to falsify the century-old, single particle (Tyndall or Rayleigh, and Mie) scattering hypotheses, which assumed that the colour comes from wavelength-dependent light scattering properties of isolated, spatially uncorrelated scatterers [3,[7][8][9]16]. But two-dimensional Fourier power spectra of TEM images lack the resolution to account for the variation in reflectance features of these complex three-dimensional nanostructures [3,[7][8][9][17][18][19][20]. They also suffer from artefacts owing to EM sample shrinkage and others related to analysing a finite-thickness (approx.…”

Concordant evolution of plumage colour, feather microstructure and a melanocortin receptor gene between mainland and island populations of a fairy–wren

“…Moreover, melanin granules appeared to be incorporated directly into the spongy layer in black males, whereas they were located in the centre of the barb beneath the spongy layer in blue males. Thus, the thick, melanized cortex of black males' feather barbs probably reduces the amount of light that can penetrate the barb (Finger 1995;Shawkey et al 2004), while the layer of melanin granules just beneath the cortex absorbs much of the light that does penetrate the barb before it Proc. R. Soc.…”