Aggressive signaling is an important component in animal communication, as it provides an efficient mechanism for settling conflicts over resources between competitors. In songbirds, a number of singing behaviors have been proposed to be aggressive signals used in territory defense, including song rate. Although aggressive signaling in songbirds has received considerable research attention, adequate evidence for most putative aggressive signals is not available. In this study, we experimentally investigated whether the song rate of male wood warblers (Phylloscopus sibilatrix) is a signal of their aggressive intent in male-male interactions. We found that males responded differentially to simulated territorial intrusions depending on the song rate of an intruder. Moreover, males that continued to sing during territorial contests increased their song rates, and this behavior predicted the strength of aggressive escalation by the signaler. These results suggest that song rate is an aggressive signal during male-male interactions in the wood warbler. We also found high intra-individual repeatability in the strength of aggressive response to simulated intrusions, likely reflecting differences in personality (aggressiveness) or quality of male wood warblers. We conclude that changes in singing rate may be an efficient mechanism of signaling immediate shifts in motivation of signalers during territorial contests, especially in species that lack large repertoires or have simple songs.