Spotless starlings prefer spotless eggs: conspecific brood parasites cue on eggshell spottiness to avoid ectoparasites

Colouration of the uropygial secretion in starling nestlings: a possible role of bacteria in parent-offspring communication

The use of cosmetic substances in communication is widespread in animals. Birds, for instance, use their uropygial secretion as a cosmetic in scenarios of sexual selection and parent-offspring communication. This secretion harbours symbiotic bacteria that could mediate the synthesis of pigments for cosmetic colouration. Here, we investigate the association between bacteria and the conspicuous yellow secretion used by spotless starling (Sturnus unicolor) nestlings to stain their mouths, and hypothesize a possible role of bacteria in the colour production. We also experimentally explore how nestling oxidizing condition influences the microbiota, suggesting that the possible bacterial-mediated coloured secretion acts as a reliable honest signal. An antioxidant supplementation experiment, previously known to affect secretion and mouth colouration, was conducted to assess its impact on the microbial community of secretions from control and experimental siblings. Antioxidant supplementation increased richness and phylogenetic diversity of the secretion's microbiota. Moreover, the microbiota's alpha and beta diversity, and the abundance of two bacterial genera (Parabacteroides and Pseudogracilibacillus), correlated with secretion colour. These findings demonstrate that antioxidant condition influences the gland microbiota of starling nestlings, and suggest a link between bacteria and the colouration of their cosmetic secretion. Alternative explanations for the detected links between bacteria and colouration are discussed.

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

Animal coloration results from pigments, nanostructures, or the cosmetic use of natural products, and plays a central role in social communication. The role of cosmetic coloration has traditionally been focused in scenarios of sexual selection, but it could also take place in other contexts. Here, by using spotless starlings (Sturnus unicolor) as a model system, we explore the possibility that nestlings cosmetically use their intensely yellow-colored uropygial secretion to signal their genetic and/or phenotypic quality. 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 color of secretion after rubbing with them nestlings' gape, and (iii) gape and skin colorations correlated positively with that of secretion. Furthermore, we found that (iv) secretion coloration has a genetic component, and (v) associated positively with Vitamin E supplementation and (vi) with plasma carotenoid concentration, which highlights the informative value of nestling secretion. Finally, (vii) coloration of begging-related traits and of secretion of nestlings predicted parental feeding preferences. Consequently, all these results strongly suggest that the cosmetic use of colored uropygial secretion might also play a role in parent-offspring communication, complementing or amplifying information provided by the flamboyant colored gapes and skin of nestlings. The use of makeups by offspring for communication with relatives has been scarcely explored and we hope that these results will encourage further investigations in birds and other taxa with parental care.

Cosmetic coloration of cross-fostered eggs affects paternal investment in the hoopoe (Upupa epops)

The signalling hypothesis suggests that avian eggshell coloration is a sexually selected female signal advertising her quality to its male partner, thereby stimulating his provisioning rate. This hypothesis has been tested for structural eggshell pigments, but not for cosmetic colorations, such as that produced by the uropygial secretion on eggshells. During the breeding season, female hoopoes (Upupa epops) host in their uropygial glands symbiotic bacteria. Females actively smear the eggshells with their secretion, protecting embryos from pathogenic trans-shell infections and changing eggshell coloration. Because the colour of the secretions is related to their antimicrobial potential, cosmetic eggshell coloration may act as a cue or even as a post-mating sexually selected signal if it affects male provisioning rates. To experimentally test this hypothesis, we cross-fostered already-smeared clutches between hoopoe nests, and quantified male feeding behaviour to females before and after the experiment. This approach allows disentanglement of the effects of female quality and of egg coloration on male investment. In accordance with the hypothesis, males adjusted their provisioning rate to the eggshell cosmetic coloration. This is, to our knowledge, the first experimental demonstration that egg colour stained with uropygial secretion could act as a post-mating sexual signal of female quality to males.