Made-up mouths with preen oil reveal genetic and phenotypic conditions of starling nestlings

Animal colouration are due to pigments, nanostructures, or to the cosmetic use of natural products, and plays a central role in social communication. The role of cosmetic colouration has traditionally been focussed on scenarios of sexual selection, but it could also be used in other contexts. Here, by using spotless starling (Sturnus unicolor) as a model system, we explore the possibility that nestlings cosmetically used their intensely yellow coloured uropygial secretion to signal their genetic and/or phenotypic quality to their parents. In agreement with the hypothetical cosmetic use of the uropygial secretion, (i) video recorded nestlings collected secretion with the bill at the age of feathering, (ii) cotton swabs turned to the colour of secretion after rubbing with them nestlings’ gape, and (iii) gape and skin colourations correlated positively with that of secretion. Furthermore, we found that (iv) secretion colouration has a genetic component, and (v) associated positively with V...