Mechanisms of evolutionary change in structural plumage coloration among bluebirds (Sialiaspp.)

Abstract

Combinations of microstructural and pigmentary components of barbs create the colour displays of feathers. It follows that evolutionary changes in colour displays must reflect changes in the underlying production mechanisms, but rarely have the mechanisms of feather colour evolution been studied. Among bluebirds in the genus Sialia, male rump colour varies among species from dark blue to light blue while breast colour varies from blue to rusty. We use spectrometry, transmission electron microscopy and Fourier analysis to identify the morphology responsible for these divergent colour displays. The morphology of blue rump barbs is similar among the three species, with an outer keratin cortex layer surrounding a medullary 'spongy layer' and a basal row of melanin granules. A spongy layer is also present in blue breast barbs of mountain bluebirds Sialia currucoides and in rusty breast barbs of western Sialia mexicana and eastern bluebirds Sialia sialis. In blue barbs melanin is basal to the spongy layer, but is not present in the outer cortex or spongy layer, while in rusty barbs, melanin is present only in the cortex. The placement of melanin in the cortex masks expression of structural blue, creating a rusty display. Such shifts in microstructures and pigments may be widespread mechanisms for the evolutionary changes in the colours of feathers and other reflective structures across colourful organisms.

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.…”

“…90 nm), low-resolution, two-dimensional slice of a three-dimensional nanostructure, such as aliasing, binning, etc. Nevertheless, most studies of avian structural colours have used twodimensional electron microscopy to characterize their underlying three-dimensional biophotonic nanostructures [3,[7][8][9]14,[17][18][19][21][22][23][24][25]. However, fundamental uncertainty remains about the exact organization of these three-dimensional amorphous feather barb nanostructures.…”

Kingfisher feathers – colouration by pigments, spongy nanostructures and thin films

“…Birds possess various pigment classes, for instance carotenoids, pterins, porphyrins, psittacofulvins and melanins (McGraw, 2006;Hill and McGraw, 2006), and various mechanisms of structural colouration, namely thin films, multilayers, photonic crystals, keratin spongy nanostructures and nanofibres (e.g. Durrer, 1977;Shawkey et al, 2003;Shawkey et al, 2006;Yoshioka et al, 2007;Doucet and Meadows, 2009;Prum et al, 2009;Stavenga et al, 2010;D'Alba et al, 2011). The predominant location of colouration is the feathers, often either the barbs or the barbules.…”

Electron tomography, three-dimensional Fourier analysis and colour prediction of a three-dimensional amorphous biophotonic nanostructure

“…Previous twodimensional Fourier predictions of the hue values for spongy medullary keratin have not been very accurate, deviating from measured values by as much as 70 nm (Prum et al 1998b(Prum et al , 1999Shawkey et al 2003). Furthermore, shapes of predicted spectra rarely match measured curves (Doucet et al 2004;Shawkey et al 2006). The substantially higher accuracy of colour predictions based on these three-dimensional tomography data indicates that a complete threedimensional description of the nanostructure is extremely useful in understanding and modelling structural colour production, particularly for amorphous photonic structures.…”