Growing evidence that female ornaments and armaments may be important for female reproductive success suggests that a reevaluation of the costs of these potentially sexually selected traits is also necessary. Here, I examine whether intrasexual aggression, a trait favored during direct female–female competition for nesting sites in tree swallows (Tachycineta bicolor), is costly in terms of the quantity or quality of offspring. I compared measures of female aggressiveness to clutch size, and I also cross-fostered offspring just after hatching to explore a possible causal link between female aggression and nestling mass, an established proxy for offspring quality. High levels of aggression in females were not associated with the quantity of offspring, but instead more aggressive females had offspring of lower quality. While several causal factors appear to influence offspring quality, the mechanism most consistent with this cost of aggression is a trade-off between female aggression and aspects of maternal care. Site differences may create variation in how selection shapes female aggression, but the finding that more aggressive females had lower-quality control offspring indicates that this cost may work counter to selection favoring aggressive behavior in the context of competition over nestboxes.