One factor hypothesized to influence the reproductive behavior of individuals is the degree to which reproductive efforts are synchronized with others in the population. We asked whether the timing of a pair's breeding cycle, relative to cycles of pairs on neighboring territories, affected rates of extrapair mating over 2 years in a Wyoming population of house wrens (Troglodytes aedon). Extrapair young (identified using 5 microsatellite loci) occurred in 74% of nests of 19 pairs whose cycles began later than cycles of one or more neighbors compared to only 26% of nests of 27 pairs whose cycles began earlier than, or simultaneously with, cycles of all neighbors. Extrapair offspring occurred in 65% of 17 nests belonging to males who initially settled and began nesting early relative to neighbors but who were forced to renest late after we removed their first mates. Rates of cuckoldry were not significantly different for forced-late and naturally late males. Our experimental approach controlled for possible effects of male quality, clearly demonstrating an effect of timing of breeding on extrapair mating activity.

Sex-allocation theory predicts that females should preferentially produce offspring of the sex with greater fitness potential. In socially monogamous animal species, extra-pair mating often increases the variance in fitness of sons relative to daughters. Thus, in situations where offspring sired by a female's extra-pair mate(s) will typically have greater fitness potential than offspring sired by the within-pair mate, sex-allocation theory predicts that females will bias the sex of offspring sired by extra-pair mates towards male. We examined the relationship between offspring sex and paternity over six breeding seasons in an Illinois population of the house wren ( Troglodytes aedon ), a cavity-nesting songbird. Out of the 2345 nestlings that had both sex and paternity assigned, 350 (15%) were sired by extra-pair males. The sex ratio of extra-pair offspring, 0.534, was significantly greater than the sex ratio of within-pair offspring, 0.492, representing an increase of 8.5 per cent in the proportion of sons produced. To our knowledge, this is the first confirmed report of female birds increasing their production of sons in association with extra-pair fertilization. Our results are consistent with the oft-mentioned hypothesis that females engage in extra-pair mating to increase offspring quality.