Abstract: Conspecific aggression has been extensively studied in males, but its evolutionary role and function in female behavior have until recently received comparatively little attention.  It has been hypothesized that female aggression in socially monogamous species plays a role in the maintenance of the mating system and/or the protection of paternal investment. These hypotheses generate contrasting behavioral predictions about the relative level of aggression breeding females will display towards male versus female conspecifics. In this experiment, we asked whether female dark-eyed juncos (Junco hyemalis) are more aggressive towards male or towards female conspecific intruders.  Nest intrusions were simulated by presenting a conspecific of each sex to individual incubating females on consecutive days in random order, and quantifying the behavioral response. In contrast to previous studies investigating similar questions in different study species, there was no significant difference between the level of aggression displayed by female dark-eyed juncos towards female versus male conspecific intruders.  There was a strong correlation between intra-individual responses, indicating that females are consistent in their aggression patterns and suggesting that there may be a common underlying mechanism regulating aggression towards both sexes. Additionally, there was a strong order by sex interaction, raising further questions about the mechanisms and potential causes driving female aggression towards conspecifics.