Melanin-based coloration and immunity in polymorphic population of pied flycatcher, Ficedula hypoleuca

A specific interest in the persistence of color polymorphism in some populations of birds and other vertebrates is often linked to ideas about the signaling honesty of bright coloration. The evolution of conspicuous ornamentation could be associated with physiological costs including limitations of the immune system. The study of this process is crucial for an understanding of the maintenance of polymorphic coloration. Here we summarized the results of a study of a pied flycatcher population from the Moscow region (Russia) in 2010–2013. We experimentally induced antibody production by injecting sheep red blood cells (SRBC) and inflammatory swelling by injecting phytohaemagglutinin (PHA) after which we estimated the immune response in breeding males. We used leucocytes-to-erythrocytes and heterophils-to-lymphocytes (H/L) ratios as indicators of infectious, inflammatory processes and stress. The results showed that the feeding rates of males treated with SRBC decreased and negatively related to the intensity of their immune responses. Non-molting males of different color types did not significantly differ in antibody production. Among molting breeders, the immune response to SRBC was significantly higher in pale males than in bright ones with rich melanin-based coloration. In contrast to non-molting males, molting pale males had an increased antibody titer after immunization. The lower humoral immune response was associated with the higher H/L stress index before immunization. The change in H/L after immunization positively correlated with the intensity of the humoral immune response. As opposed to humoral immunity, we did not find any significant predictors, including coloration, molt, or their two-way interaction, to explain the variation in cutaneous inflammatory response to PHA. The results suggest that the apparent advantage of a cryptic male phenotype over a conspicuous phenotype occurring in one of two types of immune response has an impact on the maintenance of color polymorphism in the pied flycatcher.