Comparison of morphological and reflectance properties of differently coloured feathers from male Swallow Tanagers (Tersina viridis; Aves: Thraupidae)

TOMÁS BAZZANO; DIANA CARINA SKIGIN; ANA BARREIRA; LUCAS MENDICINO; NATALIA GARCÍA; MARINA INCHAUSSANDAGUE; PABLO TUBARO

Congreso; Living Light - Biological and bio-inspired optics Faraday Discussion; 2020

Noniridescent, structurally-based colouration in birds originates from the featherbarb's internal nanostructure (the keratin spongy matrix), but the presence of melaningranules and the characteristics of the barb?s cortex can affect the resulting colourproperties. Our aim is to better understand how the feather nanostructure isregulated and combined with other elements in differently-coloured plumagepatches in a structurally-coloured species. To do so, we compared the opticalproperties of the belly and back feathers of male Swallow Tanagers which appearwhite and greenish-blue, respectively, from reflectance measurements, includingmicro-spectrophotometric measurements on the spongy matrix. We also analyzedelectron microscopy images of the barbs to investigate the association betweenthe spectral response and the feathers? morphology. The reflectance spectrameasured directly on the belly and back plumage patches of museum specimens showedthat both have a reflectance peak around 550 nm, but the reflectance spectrumis much less saturated in the belly patch, which is white to the human eye. However,a subtle green colour can be observed in the tips of feathers extracted fromthe belly. We found that the distal tip of the feather barbs of both, back andbelly feathers, have a similar spongy matrix with almost equivalent values ofsize and interspacing of the air voids in the nanostructure. The spongy matrixfrom both types of feather barbs produced similar reflectance spectra with a peakaround 530-550 nm. Belly feathers lack pigments and the morphology along theirbarbs changes considerably, with an almost complete absence of spongy matrix anda change in shape towards the feather rachis which is translated into a subtlecolour change, going from light greenish towards the tip to white closer to thefeather rachis. Instead, we observed deposition of melanin granules underneaththe spongy matrix in the back feathers resulting in a much saturated colourationwhich was consistent along the barbs? length. Overall, our results suggest thatthe drastic colour differences between the white and greenish-blue plumage ofmale Swallow Tanagers is mostly due to the differential deposition of melaninand a reduction of the spongy matrix in some parts of the belly feather barbs,and not a result of changes in the periodicity of the spongy matrix. Thissuggests that changes in colouration among plumage patches, and likely colour evolutionarytransitions, of structurally-coloured bird species can be mediated through thedifferential deposition of pigments instead of alterations of the periodicityand morphology of the colour-producing nanostructure itself.