Egg spottiness reflects female condition, physiological stress, and ornament expression in a common rallid species

Eggshell pigmentation is generated by 2 major pigments, biliverdin and protoporphyrin. The latter is mostly deposited in red, brown, and black egg spots and it has been hypothesized that greater expression of egg spottiness (as measured by the number, area, and coloration of spots) may act as an honest signal of female quality for males (sexual signaling hypothesis, SSH). The important assumption of the SSH is that eggshell pigmentation correlates with phenotypic and genetic components of female quality, although phenotypic quality of females may also be under environmental control. The aim of this study was to test for the associations of protoporphyrin-based egg pigmentation with both phenotypic and genetic female traits and environmental variables (microhabitat and urbanization) in a common rallid species, the Eurasian Coot (Fulica atra). We found that the total number of egg spots was positively associated with female condition (size-corrected body mass) and expression of a putative bare-part ornament (frontal shield). The same measure of spottiness negatively correlated with the level of physiological stress in females. No evidence was found for associations between egg spottiness and genetic traits in females (neutral heterozygosity and polymorphism of pathogen recognition receptors, the Major Histocompatibility Complex), but there was a linear increase in the expression of egg spottiness over the breeding season, which may suggest that it is regulated by food availability. Our study indicates that protoporphyrin-based pigmentation of eggs reflects female phenotypic traits (condition, stress, and ornament expression) in the Eurasian Coot, although it remains to be established whether it plays any signaling role and whether it is driven by sexual selection in this species.LAY SUMMARY There is equivocal support for direct associations between maternal quality and deposition of protoporphyrin pigmentation (dark spots and blotches) in avian eggshells. Research on protoporphyrin eggshell pigmentation has primarily focused on a single avian order (Passeriformes). We examined associations of protoporphyrin-based eggshell pigmentation with female phenotypic and genetic traits in a non-passerine species, the Eurasian Coot. Deposition of protoporphyrin in eggshells (total number per area of egg spots) positively correlated with female condition and expression of a putative bare-part ornament (frontal shield), while it was negatively associated with the level of physiological stress. Protoporphyrin-based eggshell pigmentation acts as a reliable signal of female phenotypic (but not genetic) traits in the Eurasian Coot. Signaling properties of protoporphyrin-based egg coloration are likely to largely differ between different evolutionary lineages of birds.