Hoopoe (Upupa epops) male feeding effort is related to female cosmetic egg colouration

Avian eggshell colouration might function as a post-mating sexually selected signal of female quality, influencing male parental investment and, hence, reproductive success. This hypothesis has been tested for intrinsic eggshell pigments as biliverdin (blue-green colouration) and/or protoporphyrin (brown coloured spots), but never for colourations applied post-laying. Post-laying staining colouration due to, for instance, uropygial secretion of the female could reflect its phenotypic properties and, thus, might be a cue for male investment in reproduction. In hoopoes, the uropygial gland of incubating females hosts symbiotic bacteria that are responsible for the brown colour of their uropygial secretion and of the eggshells, as they cover their bluish-grey eggshells with gland secretion after laying. The secretion protects embryos from pathogenic trans-shell infections and, thus, egg colouration may function as a cue or even as a post-mating sexually selected signal of antimicrobial pot...