Carotenoid Plasma Concentration, Immune Profile, and Plumage Ornamentation of Male Barn Swallows (Hirundo rustica). 
Carotenoids exert immunomodulating, immunostimulating, and antioxidant actions in mammals and are major determinants of coloration in animals.  Honest advertisement models of sexual selection propose that male ornaments, including coloration, are reliable indicators of male quality.  Because of their simultaneous effects on male coloration and immunity, carotenoids might mediate the hypothesized relationship between the expression of epigamic coloration and parasitism in vertebrates.  We analyzed the relationship between immune profile and concentration of lutein, the most abundant carotenoid in the plasma of male barn swallows (Hirundo rustica).  Consistent with our predictions, lutein plasma concentration was negatively correlated with gamma-globulin plasma levels and concentration of selected leukocyte types in peripheral blood, suggesting that, to exert immune function, carotenoids are taken up from plasma, thus becoming unavailable for epigamic signaling.  The coloration of red feathers of the throat of adult males was positively related to plasma concentration of lutein, but not with immunologic variables, consistent with the idea that more brightly colored males do not pay a larger immunological cost for their coloration compared with less brightly colored males.  Length of male tail ornaments, which is currently under directional sexual selection, was positively correlated with lutein plasma levels.  In species where carotenoids limit immune function, demands for pigments for sexual signaling might compete with those for immunity, thus generating a mechanism that enforces honesty on the signal.