Katarzyna Janas, Anna Łatkiewicz, Andrew Parnell, Dorota Lutyk, Julia Barczyk, Matthew D. Shawkey, Lars Gustafsson, Mariusz Cichon, and Szymon M. Drobniak
The costs associated with the production and maintenance of colour patches is thought to maintain their honesty. Although considerable research on sexual selection has focused on structurally coloured plumage ornaments, the proximate mechanisms of their potential condition-dependence, and thus their honesty, is rarely addressed, particularly in an experimental context. Blue tit (Cyanistes caeruleus) nestlings have UV-blue structurally coloured tail feathers, providing a unique opportunity for investigation of the causes of variation in their colour. Here, we examined the influence of early growing conditions on reflectance and structural properties of UV-blue coloured tail feathers of blue tit nestlings. We applied a two-stage brood size manipulation to determine which stage of development more strongly impacts the quality of tail feather colouration and microstructure. We used small angle X-ray scattering (SAXS) and electron microscopy to characterize nano- and micro-scale structure of tail feather barbs. Nestlings from the broods enlarged at a later stage of growth showed a sex-specific rectrix development delay, with males being more sensitive to this manipulation. Contrary to predictions, treatment affected neither the quality of the barb’s nanostructures nor the brightness and UV chroma of feathers. However, at the micro-scale, barb’s keratin characteristics were impaired in late-enlarged broods. Our results suggest that nanostructure quality, which determines UV-blue colour in tail feathers, is not sensitive to early rearing conditions. Furthermore, availability of resources during feather growth seems to impact the quality of feather microstructure more than body condition, which is likely determined at an earlier stage of nestling growth.