Data from: Increased extra-pair paternity in broods of aging males and enhanced recruitment of extra-pair young in a migratory bird

Despite keen interest in extra-pair mating in birds, its adaptive
significance remains unresolved. Here, we use a multi-year dataset to test
whether traits of a female's social mate influence her propensity to
produce extra-pair offspring in a population of house wrens, and whether
producing extra-pair young has consequences for a female's fitness
through effects on offspring survival. Females were most likely to produce
extra-pair offspring when paired with old males and when paired with males
on poor-quality territories, although this latter effect was marginally
non-significant. Among offspring, the cutaneous immunity of within-pair
young decreased as the age of their sires increased, but cutaneous
immunity of extra-pair young was not affected by the age of their
extra-pair sires or by the age of the males rearing them. Extra-pair
offspring were more likely than within-pair offspring to return as
breeding adults to the local population, with extra-pair sons being more
likely to return as a breeder for multiple years. Our findings support the
hypothesis that females produce extra-pair offspring to enhance their
inclusive fitness beyond what they are capable of given the male with
which they are socially paired.