Testosterone levels and gular pouch coloration in courting magnificent frigatebird (Fregata magnificens): variation with age-class, visited status and blood parasite infection.

Male magnificent frigatebirds (Fregata magnificens) possess a seasonally expressed skin ornament, namely the red and inflatable gular pouch, and are, therefore, a convenient model for the study of some theories related to the evolution of possible testosterone-dependent sexual skin coloration. Here we report the findings of a study performed over four consecutive mating seasons in the Mexican national park Isla Isabel. We investigated differences in testosterone level and gular pouch coloration in courting males in relation to the categories: age-class, visited status and blood parasite infection. Gular pouch color saturation increased with age-class. Investigated frigatebirds were infected only with Haemoproteus iwa (Haemosporida, Haemoproteidae), with an overall prevalence infection of 15.5%. Prevalence of the infection increased with birds' age-class. Testosterone levels were significantly higher in infected males, who also had lighter colored gular pouches. In non-infected males, those visited by a female had higher testosterone levels than non-visited males. Gular pouch lightness and redness were negatively correlated but only redness in non-visited non-infected males was positively correlated with testosterone levels. Gular pouch saturation in visited and infected males was positively correlated with body mass, which also increased with age-class. Mated males had lower testosterone levels and lighter, less red and saturated gular pouch coloration than courting males. In summary, we found that coloration of the male skin ornament could reflect age-class (saturation), parasite infection (lightness) and mated status (all), together with indications of condition and testosterone dependency of ornament expression.