Natural extrapair paternity matches receptivity patterns in unguarded females: evidence for importance of female choice

While observational and experimental field studies have demonstrated that extrapair paternity is associated with a variety of male traits, there are still few published studies associating measures of paternity with behavioural preferences of females for extrapair males. In this sense, the majority of studies of extrapair paternity have failed to separate male competition from female choice. In this study we compared patterns of extrapair paternity in western bluebirds, Sialia mexicana, to previously published data on extrapair mating preferences from the same population. Results from previous experiments, where females’ mates were prevented from guarding or otherwise interfering with extrapair mating, accurately predicted patterns of paternity at 256 unmanipulated nests examined over a 6-year period. Specifically, as predicted based on which males were accepted as extrapair copulation partners, genetic extrapair sires were older but not larger than a female’s social mate. At the same time, relatively older extrapair males were not proportionally more successful, in terms of proportion of offspring sired, than younger males. Together these findings indicate that female choice for older males is expressed, but male competition, sperm competition and intersexual conflict also influence the magnitude of paternity. We suggest that experimental approaches, combined with measures of paternity under unmanipulated conditions, provide important opportunities to test the potential role of male versus female behaviours as selective factors in extrapair fertilizations.