Parasite infection, but not immune response, influences paternity in western bluebirds

Parasites can impose heavy costs on their hosts, and females may benefit from selecting mates that are parasite resistant and/or have a stronger immune response. Trade-offs between immune response and sexual signaling have been proposed as a mechanism to ensure signal honesty. Much of the work on sexual signaling and immune response does not consider parasites directly and thus cannot confirm whether a stronger immune response necessarily results in lower parasite burdens. Also, immunity is costly, and these costs can lower the overall fitness of individuals with too strong of an immune response. Males with immune responses of intermediate strength are therefore expected to be preferred by females and have the highest reproductive success. We tested whether immune response and blood parasite loads relate to sexual signaling and mating preferences in western bluebirds (Sialia mexicana). Immunity did not predict male reproductive success when considering either within- or extra-pair offspring, although a stronger immune response was correlated with lower parasite loads. However, uninfected males were more likely to sire extra-pair offspring than males infected with avian malaria. Thus, females were more likely to mate with uninfected males but not necessarily males with a stronger immune response. Our results may indicate that females select parasite-resistant males as mates to gain resistance genes for their offspring or that infected males are less likely to pursue extra-pair copulations.