Abstract: Previous studies have shown that male dark-eyed juncos ( Junco hyemalis ) with elevated T exhibit higher fitness than control males due to extra-pair fertilizations. Potential benefits associated with higher levels of T in males may, however, be offset by deleterious effects in females. In the female dark-eyed junco, previous studies have suggested that females are behaviorally insensitive to T during the incubation stage. In this study I asked whether experimentally elevated T affects maternal aggression towards conspecifics during incubation and nestling brooding in an effort to determine the fitness consequences associated with elevated T in females. Implanted females were presented with a conspecific male at the nest and attack intensity was measured for fifteen minutes. Brooding behaviors of females and male parental effort were observed for four hours when nestlings were three days in age. High T females spent less total time spent on the nest and less time brooding, demonstrating female sensitivity to T during the nestling stage as well as evidence for the potential constraint on the evolution of high T in males. Increased T had no effect on maternal aggression, providing additional support for female insensitivity to T during incubation.