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 potential in hoopoes. In a wild hoopoe population breeding in nest boxes in Spain, we test this hypothesis by exploring whether egg colour predicts male parental investment. In accordance with the hypothesis, we found that the amount of food provided by males to incubating females was higher in nests with less saturated eggshell colours. This relationship was affected by female body condition. High quality females in terms of body condition and/or in secretion colour obtained better males in terms of provisioning effort during incubation. Given that eggshell saturation is negatively related to density of bacterial symbionts in uropygial secretions, one possibility is that males may regulate their parental investment in accordance to the expected characteristics of mutualistic bacteria hosted in uropygial glands and deposited on eggshells. We discuss alternative explanations for our results, concluding that the post-mating sexual selection hypothesis is the most likely but experimental modification of egg colour is needed to test it further.