Unravelling the genetic basis of phenotypic variation among individuals is an important step in our understanding of evolution. Recent studies of innate immune genes, such as β -defensins, revealed that these genes had high levels of polymorphism. However, researchers have yet to quantify the effects of such variability on immune responses and fitness-related traits in wild populations. In this study, we assessed how the variability at six avian β -defensin (AvBD) genes was linked to an immune function and reproductive success in adult tree swallows (Tachycineta bicolor). We investigated the links between genetic variations using single nucleotide polymorphisms at AvBD genes, immune function as the bacterial killing ability (BKA) and fledging success. We assessed how female immunogenetics were linked to the presence of eggshell bacteria in their clutches and hatching success. We found weak associations between the presence of AvBD genes, BKA and eggshell bacteria. Our results suggested that homozygosity at some loci may be advantageous for defence against bacteria. Variability at β -defensin genes was not related to either hatching or fledging success. BKA of parents was positively linked with fledging success. More studies are needed to assess whether or not β -defensin genes are significantly affecting fitness-related traits in wild populations.