Seasonal variation in the relationship between cellular immune response and badge size in male house sparrows (Passer domesticus)

The immunocompetence handicap hypothesis postulates that secondary sexual t raits are honest signals of male quality because steroid hormones (such as corticosteroids and sex steroids), which are supposed to favor the developm ent of secondary sexual traits, may also have immunosuppressive effects. Ce rtain secondary sexual traits are not only used as mate choice signals but also play a role as badges of status. In the house sparrow (Passer domestic us), males have a bib of black feathers which is used both as a signal of s ocial status in male-male interactions and by females when choosing a mate. We investigated the relationships between bib size and cellular immune res ponse in male house sparrows during and outside the reproductive season. Ma les with large badges were found to have lower levels of immunocompetence, as assessed using a T-cell-mediated immunity assay, during the reproductive season, as predicted by the immunocompetence handicap hypothesis. Converse ly, in November, the correlation between badge size and cellular immune res ponse was positive, possibly reflecting the better access to trophic resour ces of large-badged dominant males in winter flocks.