Free-ranging birds have been used extensively to examine causal relationships between short-term changes in plasma testosterone (T) and the expression of androgen-dependent social behavior such as singing and aggressive responses to conspecific males. Due to conflicting findings, the existence and especially the generality of these relationships across species remain debated. Our work investigates this question using seasonally breeding, socially monogamous, and territorial Sonoran Desert (USA) male passerines as experimental models. During the breeding season, an injection of gonadotropin-releasing hormone or of luteinizing hormone to these birds rapidly increases plasma T, indicating that plasma T secretion in unmanipulated birds is not maximum. Yet an acute social challenge consisting of conspecific song playback with or without live decoy rapidly stimulates aggressive responses, but does not also elevate plasma T. Furthermore, there is no correlation either between the duration of this challenge and plasma T or between individual differences in plasma T and the expression of aggressive responses. These observations do not provide support to the hypothesis that social challenge rapidly upregulates plasma T or that plasma T levels determine individual differences in behavior expression. To investigate whether plasma T is related to the expression of aggressive responses on a longer-term basis, males were caught and restrained for 30 min to decrease plasma T, and then released on territory. One day after release, plasma T remained well below that of control (naïve) birds, yet the two groups of birds responded similarly to social challenge. Thus, depletion of plasma T for 24⁺ hours was not associated with attenuated behavioral response to social challenge. Taken collectively, our data do not suggest in the study species that circulating T levels during the breeding season, when these levels are seasonally elevated, control aggressive behavior or rapidly respond to social interactions. Despite considerable effort and research on many species exhibiting various life history characteristics, the specific parameters that determine whether such relationships are observed remain difficult to predict.