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

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

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

Non iridescent, structurally-based colouration in birds originates from the feather barb´s internal nanostructure(the keratin spongy matrix), but the presence of melanin granules and the characteristics of the barb?s cortex canaffect the resulting colour properties. Our aim is to better understand how the feather nanostructure is regulatedand combined with other elements in differently-coloured plumage patches in a structurally-coloured species.To do so, we compared the optical properties of the belly and back feathers of male Swallow Tanagers (whichappear white and greenish-blue, respectively) from reflectance measurements including micro-spectrophotometricmeasurements on the spongy matrix. We also analyzed electron microscopy images of the barbs to investigatethe association between the spectral response and the feathers? morphology. The reflectance spectra measureddirectly on the belly and back plumage patches of museum specimens showed that both have a reflectancepeak around 550 nm, but the reflectance spectrum is much less saturated in the belly patch, which is white to thehuman eye. However, a subtle green colour can be observed in the tips of feathers extracted from the belly. Wefound that the distal tip of the feather barbs of both, back and belly feathers, have a similar spongy matrix withalmost equivalent values of size and interspacing of the air voids in the nanostructure. The spongy matrix fromboth types of feather barbs produced similar reflectance spectra with a peak around 530-550 nm. Belly featherslack pigments and the morphology along their barbs changes considerably, with an almost complete absence ofspongy matrix and a change in shape towards the feather rachis which is translated into a subtle colour change,going from light greenish towards the tip to white closer to the feather rachis. Instead, we observed deposition ofmelanin granules underneath the spongy matrix in the back feathers resulting in a much saturated colourationwhich was consistent along the barbs? length. Overall, our results suggest that the drastic colour differencesbetween the white and greenish-blue plumage of male Swallow Tanagers is mostly due to the differentialdeposition of melanin and a reduction of the spongy matrix in some parts of the belly feather barbs, and nota result of changes in the periodicity of the spongy matrix. This suggests that changes in colouration amongplumage patches, and likely colour evolutionary transitions , of structurally-coloured bird species can be mediatedthrough the differential deposition of pigments instead of alterations of the periodicity and morphology of thecolour-producing nanostructure itself.