Data from: 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.