Vocal performance refers to the proficiency with which a bird sings songs that are challenging to produce, and can be measured in simple trilled songs by their deviation from an upper bound regression of frequency bandwidth on trill rate. Here, we show that male swamp sparrows (Melospiza georgiana) increase the vocal performance of individual song types in aggressive contexts by increasing both the trill rate and frequency bandwidth. These results are the first to demonstrate flexible modulation by songbirds of this aspect of vocal performance and are consistent with this signal feature having a role in aggressive communication.