Honest signaling by structurally based plumage coloration in the eastern bluebird (Sialia sialis)

Honest signals are traits that reflect aspects of individual quality. Because these traits are costly to produce and maintain, the bearer of the trait cannot falsely signal its quality. Previously, structural plumage coloration has been considered a signal of body condition, parental effort, and immune strength. In this study, we tested whether the blue intensity of the structural plumage of breeding male and female eastern bluebirds (Sialia sialis) is an honest signal of these qualities. We found positive correlations between blue intensity and both feather growth rates (an indicator of nutritional condition at molt) and current body condition in both sexes. In addition, we found positive relationships between blue intensity and chick mass in both males and females, suggesting that blue intensity is an honest signal of parental quality. We observed no correlation between blue intensity and plasma natural antibody levels; however, males that reared the heaviest chicks showed the greatest oxidative stress levels, as determined by plasma lipid peroxidation. Unlike males, females with high levels of lipid peroxidation had low natural antibody levels. These correlations suggest that sex-specific roles in reproduction may elicit different trade-offs among parental effort, immunity, and oxidative stress. Therefore, in this study, we provide evidence of honest signaling by structural plumage coloration and trade-offs between reproduction and self-maintenance in the eastern bluebird