Abstract: One of the most studied hormones in relation to reproductive behaviors in vertebrate animals is testosterone (T). The effects of T on the male dark-eyed junco ( Junco hyemalis ) are well known; however the effects on females of the same species are poorly understood. In order to asses the overall fitness effects of T in female juncos and thus to help explain how natural levels evolved in both sexes, I compared intensity of nest defense at two stages of reproduction, in two groups of females, one implanted with T (n=48) and on implanted with empty implants (n=53 control or C-implanted females). Implanted females were presented with a taxodermic eastern chipmunk (Tamias striatus) mount and the intensity of attack was measured by number of hits, dives and nest checks performed. Overall nest defense increased through the nesting cycle with females attacking significantly more at the nestling stage than the egg stage. All aspects of nest defense (hits, dives and nest checks) were highly correlated with each other and overall, I observed fewer nest defense behaviors by T-implanted females. This result could possibly be explained by a decrease in ‘decision-making ability' in females (McGlothlin et al. 2004), an inability to assess reproductive value of young, or a decreased fear response due to increased T levels (King et al. 2005).