Male Bachman's Sparrows (Peucaea aestivalis) have large vocal repertoires containing many song types and multiple categories of songs and calls. We examined the territorial defense function of the vocal repertoire of male Bachman's Sparrows by quantifying singing behaviors and aggressiveness in response to a simulated territorial intrusion. We compared vocal and other behaviors between the intrusion and post-intrusion periods, compared more aggressive males (attackers) with less aggressive males (non-attackers), and tested for signals that predict attack. During intrusion, subjects switched among their song types at higher rates, however, song switching did not differ between more and less aggressive males. More aggressive males sang more low-amplitude “whisper songs.” In line with previous studies of aggressive signaling in songbirds, Bachman's Sparrows appear to use low-amplitude song to threaten rivals. The eavesdropping avoidance hypothesis predicts that low-amplitude songs should have acoustic traits besides amplitude to minimize signal propagation and transmission range. The whisper songs of Bachman's Sparrows are quieter versions of primary song types. Thus, Bachman's Sparrow may be a useful species for testing other predictions of the eavesdropping avoidance hypothesis, and for testing whether reduced amplitude is sufficient to reduce the costs of eavesdroppers.