Within-pair young are more immunocompetent than extrapair young in mixed-paternity broods of the red bishop

Two competing hypotheses are commonly used to explain matings with multiple males in female birds. The genetic benefit model sees extrapair copulations as a means to enhance offspring viability, while the fertility-insurance hypothesis states that females copulate with multiple males to guarantee fertilization of eggs. According to the genetic benefit hypothesis, extrapair young are expected to have a genetic advantage over their maternal half-sibs raised in the same brood. A quantifiable measure for offspring quality is the immunocompetence, the ability of an individual to cope with infections. In this study, we used the phytohaemagglutinin induced skin-swelling response technique to test whether the cell-mediated immune reaction differs between extrapair and within-pair nestlings from mixed-paternity broods in the polygynous red bishop, Euplectes orix. The strength of the nestlings' immune response was positively linked to general health status and immunological condition of their genetic fathers, indicating a heritable component of nestling immunocompetence. We found that extrapair young were actually significantly less immunocompetent than within-pair young from the same nest, a result that does not lend support to the genetic benefit hypothesis. We suggest that female red bishops mated to successful high-quality males engage in extrapair copulations to insure against temporary infertility in their social mates caused by sperm depletion because of frequent copulations with their multiple female partners. This result, however, was significant only in the hotter of 2 years, indicating that environmental factors can obscure variation in nestling fitness arising from female extrapair mating behaviour.