Within a network of communicating individuals, animals may gather information about the relative quality of conspecifics by eavesdropping on their signalling interactions. For territorial male songbirds, eavesdropping may be a low-cost, low-risk method for assessing the relative quality of the males around them. We used a three-speaker playback design to evaluate whether male black-capped chickadees (Poecile atricapillus) respond differently to two simulated countersinging intruders who differ only in relative features of their singing performance. We arranged three loudspeakers in an equilateral triangle at the center of playback subjects' territories. After luring males to the first loudspeaker by broadcasting non-song vocalizations, we played songs from the remaining loudspeakers to simulate a countersinging interaction between two male intruders. During the interactions, one simulated intruder consistently overlapped the songs of the other, a behaviour thought to be a signal of directed aggression in songbirds. Territorial male chickadees discriminated between the simulated intruders by preferentially approaching the loudspeaker broadcasting the overlapping signal, suggesting that males eavesdrop on other males' countersinging interactions. Male responses to playback support the idea that overlapping is a more threatening signal than being overlapped. Responses varied with the dominance status of the subject. High-ranking males approached the overlapping loudspeaker in 15 of 16 cases whereas low-ranking males approached the overlapping speaker in only 5 of 10 cases, suggesting that males of different quality may use different tactics for territorial defense.