We studied the behavior of the Dusky Bush-Tanager (Chlorospingus semifuscus; Thraupinae) in southwestern Colombia, where it seems to exhibit singing assemblages resembling leks. A lek mating system would be paradoxical in a genus noted for its active participation in mixed-species flocks, largely insectivorous diet, and stable pair bond. We used the bird's behavior and the relations between fruit availability, its diet, and its participation in mixed flocks to try to explain the social system. We found males singing in dense linear assemblages, regularly spaced along ridgetops, during early morning and late afternoon hours during much of the year, with a peak from May through August. Singing males occupied small circular territories, which usually were reoccupied the next year by the same males. The critical resource is likely the males themselves, although fruit availability may permit them to remain on territories for long periods; nesting areas are apparently far from the ridgetops. Participation in flocks occured year-round but was somewhat reduced during the singing season and in the singing hours. The evidence of pair bonds in this species, in which males participate at least in nest-site selection and feeding of young, excludes the possibility of a lek mating system in the strict sense. Nevertheless, the system of singing linear assemblies appears to have been superimposed upon the typical social system of the genus. This social system is unique in the entire nine-primaried oscine assemblage.