Though rarely mate-limited, females in a wide variety of species express traits commonly associated with mate competition in males. Recent research has shown that these competitive traits (ornaments, armaments, and intense aggression) often function in the context of female-female competition for nonsexual reproductive resources and are often positively related to reproductive success. Increased success could occur because competitive females acquire limited ecological resources (nest sites, territories, etc.) or because they pair with high quality males, that is, older, more ornamented, or more parental males. Further, males paired with aggressive/low care females may compensate by increasing their paternal efforts. Here, I examined patterns of social pairing and parental care in free-living dark-eyed juncos (Junco hyemalis), a biparental songbird. I found no detectable relationship between female competitive behavior (aggression) and male quality (age, size, or ornamentation) or male provisioning. Thus, neither of the mate choice hypotheses (females compete for males or males prefer aggressive females) was supported. Instead, these results suggest that females compete for nonsexual resources and mate quality is a secondary consideration. I also found a negative relationship between male and female provisioning rates, suggesting that partners adjust their level of parental effort in response to their partner’s efforts. 1. Introduction Though females are rarely mate-limited, they are often limited by access to other important reproductive resources such as territories, ovoposition sites, and dominance rank [1–6]. Social selection argues that competition for any important resource, sexual or nonsexual, can lead to the evolution of ornaments, armaments, intense aggression, or complex acoustic signals, hereafter referred to as competitive traits [1, 2]. Recent empirical work examining the functional consequences of female expression of competitive traits supports the predictions of social selection; females with greater degree of trait expression often have improved reproductive success [1, 2, 4, 6, 7]. This pattern is particularly interesting because in many vertebrates increased trait expression is also associated with a reduction in some forms of maternal care [8–13]. A positive relationship between trait expression and reproductive success might emerge because, as social selection would predict, competitive females are more likely to acquire high quality mates or other limited reproductive resources. However, an alternative explanation is that males
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