Indicators of physiological stress and the elaboration of sexual traits in the collared flycatcher

Stress may have consequences for the evolution of condition-dependent sexual traits. For example, stress may be related to sexual traits through immune function, and sexual traits can reflect how individuals bear the costs of stress-mediated immunosuppression. However, male traits may be directly associated with stress, and such traits would then indicate stress tolerance. Here, we present initial results for the relationship between physiological stress estimated by the levels of heat shock proteins (HSP60 and HSP70) and heterophil/lymphocyte ratio and the elaboration of sexual traits, such as forehead and wing patch size and song features in the collared flycatcher Ficedula albicollis. Males producing longer and more versatile songs had significantly higher levels of HSP70, but other traits were unrelated to stress. In general, effect sizes for the relationship between stress and sexual traits had broad confidence intervals and varied between being small and medium effects. Immunoglobulin levels, leukocyte abundance, haemoparasite prevalence, male age, and date and time effects did not affect the relationship between stress and sexual traits. These preliminary results, serving a basis for further experimental studies indicate that the relationship between sexual traits and stress does not seem to be strong, but stress may partially constrain the expression of some sexual traits.