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

MENDICINO LUCAS RODRIGO; GARCÍA, NATALIA C.; INCHAUSSANDAGUE, MARINA E.; TUBARO, PABLO L.; BAZZANO, LISANDRO TOMAS; SKIGIN, DIANA C.; BARREIRA, ANA S.

EVENTO ONLINE

Congreso; Biological and Bio-inspired optics Faraday Discussion - Living Light Edition; 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 can affect the resulting colour properties. Our aim is to better understand how the feather nanostructure is regulated and 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 which appear white and greenish-blue, respectively, from reflectance measurements, including micro-spectrophotometric measurements on the spongy matrix. We also analyzed electron microscopy images of the barbs to investigate the association between the spectral response and the feathers? morphology. The reflectance spectra measured directly on the belly and back plumage patches of museum specimens showed that both have a reflectance peak around 550 nm, but the reflectance spectrum is 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 from the belly. We found that the distal tip of the feather barbs of both, back and belly feathers, have a similar spongy matrix with almost equivalent values of size and interspacing of the air voids in the nanostructure. The spongy matrix from both types of feather barbs produced similar reflectance spectra with a peak around 530-550 nm. Belly feathers lack pigments and the morphology along their barbs changes considerably, with an almost complete absence of spongy 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 of melanin granules underneath the spongy matrix in the back feathers resulting in a much saturated colouration which was consistent along the barbs? length. Overall, our results suggest that the drastic colour differences between the white and greenish-blue plumage of male Swallow Tanagers is mostly due to the differential deposition of melanin and 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. This suggests that changes in colouration among plumage patches, and likely colour evolutionary transitions, of structurally-coloured bird species can be mediated through the differential deposition of pigments instead of alterations of the periodicity and morphology of the colour-producing nanostructure itself.