Biological significance of forced extra-pair copulations in a population of European Blackbirds

Functional explanations for forced copulation, an extreme manifestation of intersexual conflict, remain underexplored. We investigated the adaptive significance of forced extra-pair copulation (FEPC) in European blackbirds. Our results contradict the prevailing hypothesis that the primary function of FEPC is fertilization. FEPC frequency peaked at the beginning of the breeding season and declined thereafter, despite the continued presence of fertile females. Furthermore, we found no evidence that males selectively target fertile females; within the reproductive cycle, females were equally susceptible to FEPC during the pre-fertile and fertile periods. We propose that sexual aggression in blackbirds reflects competition. As predicted, the incidence of FEPC was highest early in the season, and females attempting to settle between established pairs were at higher risk of becoming victims. Females were not passive in the conflict and could significantly reduce the risk of FEPC by shifting home ranges. We conclude that FEPC is an instrument of aggression rather than an expression of sexual interest. Neighboring males attempt to deter new competitors by targeting the weaker member of a pair. For females, FEPC is part of the landscape of fear.