The histories of five colony-specific song dialects of yellow-rumped caciques (Cacicus cela vitellinus) were recorded for three breeding seasons in Panama. Dialects consisted of 5 to 8 song types shared among all male members of a breeding colony. The number of song types that occurred at more than one colony decreased with distance between colonies. Color-banded subadult and adult males frequently dispersed between years to other dialect areas where they adopted the local songs. About 78% of all song types were distinct from songs present the previous year at the same colony. During a breeding season some song types changed gradually, with all males adopting parallel changes. Gradual change in songs at colonies appeared to account for more new song types than did introduction of songs by dispersing males. Because song types changed gradually in constant directions, the cause of these changes may be cultural adaptation in which individuals benefit by adopting certain songs, rather than copy-error or cultural drift.