Vocal dialects have presented a puzzle for evolutionary biologists for nearly 50 years. However, there is still a dearth of detailed description of variation in species with dialects, especially regarding acoustic characteristics that discriminate among dialects. In this study, I show that dialects occur in the “flight whistle” song type of the Bronzed Cowbird (Molothrus aeneus). Dialects in this species occur at a very large scale, spanning hundreds of kilometers. A discriminant function analysis revealed that the initial element of the song shows greater differences among dialects than a major element in the final portion of the song. This quantitative analysis of the dialect system provides the basis for understanding how Bronzed Cowbirds recognize songs from different dialects.