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Sperm competition theory predicts that colonially breeding birds, such as many seabirds, should be exposed to high risks of sperm competition because there are many potential mating partners and severe socioecological constraints on mate-guarding behaviour. However, parentage studies have usually revealed a low frequency of extrapair paternity in colonial seabirds. Detailed studies of copulation behaviour could provide valuable insights into the causality of genetic monogamy in these species. We investigated the timing, frequency and success of extrapair and within-pair copulation attempts in the little auk, a socially monogamous and highly colonial seabird that raises only a single chick per year. We found that the majority of monitored individuals (above 60% in both sexes) engaged in extrapair copulation activities. Extrapair copulation attempts made up usually more than 10% of all copulation attempts of a particular individual but were generally unsuccessful because of female rejection behaviour. Only 2% (8/330) of all copulations with cloacal contact were extrapair copulations. Molecular parentage analysis of 64 chicks revealed only two cases of extrapair paternity, which is comparable to the proportion of successful copulations that were extrapair. We conclude that genetic monogamy seems to be maintained at a proximate level through female resistance to male extrapair copulation attempts and frequent within-pair copulations. Nevertheless, not all extrapair copulations were rejected by females, which suggests that engaging in one or a few extrapair copulations could be adaptive to females, for example as an insurance against mate infertility. [Copyright &y& Elsevier]

Animals often face trade-offs during reproduction between activities such as parental care and territory defence. In species that are socially monogamous, males and females are faced with the additional problem of coordinating their respective contributions to such activities. Here, we examined male incubation behaviour in a passerine that shows genetic monogamy and in which males incubate extensively. Male incubation is rare in North American passerines and little is known about how males defend their territories during the incubation stage and whether females compensate for lower male incubation during territorial intrusions. Blue-headed vireo males contribute 50% of incubation time during the day. We performed playbacks to simulate intrusions to males while they were on the nest and off the nest during the incubation period. On average, males took 18min to arrive at the playback location while on the nest incubating, but took less than 2min when not incubating. In addition, only 44% of the males sang while on the nest, whereas all males sang in response to the playback when off the nest. This result suggests that males probably delay territory defence until females return to the nest to avoid exposing the nest to predators. Females returned to the nest to relieve their mates from incubation duties sooner during experimental intrusions than during control periods, which also allowed males to pursue intruders sooner. Genetic monogamy may underlie this apparent cooperation and the sex-role convergence exemplified in this species. [Copyright &y& Elsevier]