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.

中文翻译：

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Hoopoe Upupa epops男性的喂养努力与女性化妆品的鸡蛋着色有关

禽蛋壳着色可能是性交后选择的女性品质信号，影响男性父母的投资，进而影响生殖成功。该假设已针对内在的蛋壳色素如biliverdin（蓝绿色）和/或protoporphyrin（棕色斑点）进行了测试，但从未进行过产后着色。由于雌性尿尿分泌引起的产后染色着色可能反映了其表型特性，因此可能是雄性生殖繁殖投资的线索。在戴胜的河豚中，正在孵化的雌性的尿道腺含有共生细菌，这些细菌导致尿道分泌物和蛋壳呈棕色，因为它们在产蛋后被腺体分泌物覆盖着蓝灰色蛋壳。分泌物可保护胚胎免受病原性跨壳感染，因此，鸡蛋着色可能是提示，甚至可以作为戴胜后戴胜的有性选择的抗菌肽潜在信号。在西班牙巢箱中繁殖的野生戴胜种群中，我们通过研究蛋色是否可以预测男性父母的投资来检验这一假设。根据该假设，我们发现，在蛋壳颜色饱和度较低的巢中，雄性为孵化雌性提供的食物量较高。这种关系受到女性身体状况的影响。就身体状况和/或分泌颜色而言，高质量的雌性在孵化期间的供应努力方面获得了更好的雄性。考虑到蛋壳饱和度与尿路泌尿道分泌物中细菌共生菌的密度负相关，一种可能性是，雄性可根据寄养在尿道腺中并沉积在蛋壳上的互惠细菌的预期特征来调节其父母的投资。我们讨论了对结果的其他解释，得出结论认为，交配后的性选择假设最有可能，但需要对蛋色进行实验性修改以进一步测试。