Abstract
Extra‐pair mating is widespread in birds, but its adaptive function remains unclear. It is often suggested that females obtain superior genes for their offspring as a consequence of extra‐pair mating, but the evidence is limited. In this study, we examined the hypothesis that extra‐pair mating provides females with offspring that have superior immune responses. We found that the T‐cell‐mediated immune response of extra‐pair young was stronger than that of within‐pair young in common yellowthroats ( Geothlypis trichas ). This paternity effect occurred when we compared all nestlings in the population, as well as in comparisons of both paternal and maternal half‐siblings. Paternal half‐siblings had a stronger immune response when they were produced with extra‐pair females than with the male's social mate, which suggests that the greater immune response of extra‐pair young was caused by nonadditive (compatible) genetic effects. However, these patterns were only significant in the colder of 2 years. Immune response was related positively to air temperature and nestlings had a stronger immune response in the warmer year. We suggest that such environmental variation could obscure the genetic benefits of extra‐pair mating.