Immune response of male barn swallows in relation to parental effort, corticosterone plasma levels, and sexual ornamentation

Life-history theory posits trade-offs between fitness components. Reproduction negatively affects physiology and immune system functioning, and the effect of this form of stress may be mediated by glucocorticosteroids. We manipulated brood size of barn swallows (Hirundo rustica) to study the effect of stress arising from reproductive effort on corticosterone levels of males. We also measured T-cell-mediated immunocompetence by intradermally injecting birds with phytohemagglutinin, which is mitogenic to T-lymphocytes. The results confirmed the prediction of a negative effect of parental effort on lymphoproliferative response. We found no covariation between immune response and corticosterone levels. Males with long tails, an ornament currently under directional sexual selection, had a relatively large T-cell response to the mitogen, consistent with models of parasite-mediated sexual selection predicting higher levels of immune defense in highly ornamented males. In addition, males with large sexual ornaments had relatively low corticosterone levels at the end of the parental period. These results can be reconciled with the hypothesis proposing a trade-off between parental activities and adaptive immunity and suggest that highly ornamented males are less exposed or less susceptible to stress arising from parental effort