The question of how honesty is maintained in animal communication is a perplexing one, especially in the context of aggressive communication, in which the interests of signalers and receivers are opposed. Relevant information for receivers in aggressive interactions includes the fighting ability of the signaler and its likelihood of attack. Signalers may, however, benefit from manipulation of signals to exaggerate this information. To address questions of signal honesty, this dissertation investigates the use of a performance signal in the song of swamp sparrows (Melospiza georgiana). Performance signals are ones that are physically demanding to perform and that therefore reveal the signaler’s physical ability. The particular measure of vocal performance examined here is vocal deviation, which refers to the ability of birds to produce trilled songs approaching a performance boundary defined by the tradeoff between song trill rate (the rate at which elements of a song are repeated) and frequency bandwidth (the range of frequencies encompassed in a song). Three questions regarding the adaptive significance of vocal performance were investigated in the swamp sparrow 1) whether vocal performance is used as a signal in male-male communication, 2) whether vocal performance should be classified as an index signal of male quality, and 3) whether vocal deviation serves as a signal of aggressive intent. Results from four studies indicate that vocal performance plays a role in male-male signaling. Signalers appear to use vocal performance as a signal and receivers attend to differences in vocal performance. Evidence also indicates that vocal performance can be classified as an index signal, because males are constrained in their ability to produce high performance song and high vocal performance is correlated with aspects of male quality, such as age and size. Finally, this dissertation does not support classification of vocal performance as a signal of aggressive intent, because vocal performance fails to predict aggressive escalation.