Acoustic divergence among populations may result in assortative mating, behavioral isolation, and speciation. In birds, the recognition of suitable mates depends to a large extent on learning, generally resulting in a tendency to discriminate against nonlocal stimuli. However, there may be geographical variation in the discrimination against nonlocal stimuli, and this may allow inferring the mechanisms behind the evolution of vocal recognition. We tested territorial males of 3 west European subspecies of reed bunting (Emberiza schoeniclus schoeniclus, Emberiza schoeniclus lusitanica, and Emberiza schoeniclus witherbyi) using song playback to determine the level of song discrimination. We found that witherbyi and, to some extent lusitanica, males largely ignored schoeniclus songs. However, witherbyi reacted less strongly to the songs of lusitanica than lusitanica did to songs of witherbyi. In contrast, schoeniclus males did not discriminate the songs of the different subspecies, reacting strongly to all. Differential territorial defense behavior suggest that intruding males with different songs do not represent the same competitive threat, and provide evidence of premating reproductive isolation among these recently evolved subspecies. The high discrimination exhibited by witherbyi and lusitanica seems associated with the high level of local adaptation. Overall, the pattern of premating reproductive isolation appears to agree more with the ecological than with the neutral genetic divergences between subspecies, suggesting that there is an ongoing process of ecological speciation in this study system.