Uropygial secretion changes mouth colouration in starling nestlings.

Cosmetic colourations of animals have been mainly studied in scenarios of sexual selection, while there has been no assessment of the partial contribution of cosmetics to the final colouration of begging-related traits. In birds, the uropygial gland is functional soon after hatching, and we experimentally investigated the effects of uropygial secretion on mouth-flange colouration in spotless starling (Sturnus unicolor) nestlings. Nestlings' flange colouration was measured with a spectrophotometer before and after being either cleaned of, or painted with, its own uropygial secretion. After cleaning, flanges were brighter, more UV and less yellow-red coloured (chroma). Instead, painting the mouth flanges of nestlings with their own uropygial secretion did not modify any of the considered colour components. Our findings therefore show that the uropygial secretion of nestlings alters their beak colouration and open the possibility for future research to investigate the role of uropygial secretion in reinforcing the signalling role of begging-related traits involved in parent-offspring communication.