Mixed-species flocks of birds are cohesive groups of two or more different species that are kept in formation by systematic behavioral interactions among the members of the flocks. Typically, mixed-species flocks form themselves around a nuclear species, which presents a specific pattern of movement during foraging, and a song that is recognized by the other species. In the understory of Neotropical forests, representatives of the family Thamnophilidae, in particular species of the genus Thamnomanes, have become adapted for the role of nuclear species in mixed-species flocks. In the present study, we describe the mixed-species flocks led by individuals of Bluish-slate Antshrike Thamnomanes schistogynus in a forest dominated by bamboo Guadua sp. in southwestern Brazilian Amazonia. We collected data within a 600 × 600 m grid subdivided into 36 1-ha plots, with a total area of 36 ha. Within this grid, we delimited the home ranges and core areas of the resident mixed-species flocks. We also identified the composition of each flocks, frequency, fidelity of its members and determined the relationship between mixed-species flocks and vegetation structure within the grid. We delimited the home ranges of eight mixed-species flocks. The mean home range, estimated using the Minimum Convex Polygon method, was 3.45 ± 0.17 ha, and estimated using the autocorrelated kernel density estimation method was 3.65 ± 0.15 ha, and the mean core area, 1.14 ± 0.03 ha. The home ranges of the mixed-species flocks led by T. schistogynus in southwestern Amazonia are among the smallest recorded in species of the genus Thamnomanes. We identified 71 different bird species in the mixed-species flocks with a mean of 41.1 ± 2.1. While the geographic distribution of the lead species T. schistogynus coincides almost exactly with that of the bamboo forests of southwestern Amazonia, our results did not indicate any systematic relationship or fidelity between the mixed-species flocks led by this species and the bamboo forest.