In a study of male song in the orange-tufted sunbird (Nectarinia osea) in an urban neighborhood in Ramat-Aviv, Israel, we discovered the occurrence of song variation on a microgeographic scale in the form of two distinct dialects with a sharp boundary between them. The main distinction between the two song dialects is the frequency of the trill, which comprises the terminal part of the song. A large difference of 2-3 kHz in the peak frequency of the trill was discovered between the two dialects, which could be easily distinguished by ear. Thirtyseven males were recorded singing the 'low' dialect and 21 birds sang the 'high' dialect. Four other birds sang both dialects or 'hybrid' songs. Along the boundary that separated the two dialect populations, neighboring birds sang different dialect songs, although they were only 20-30 meters apart. All four 'bilingual' birds occupied territories near the dialect boundary. The historical processes leading to the formation of this dialect system may result from the pattern of human settlement at the time of the establishment of this neighborhood in the early 1950's. The spatial distribution of the two sunbird dialect populations, and the apparent low dispersal rates of birds from their natal dialect area, suggest the existence of a mechanism, which currently maintains these dialects at the current boundaries.