Despite considerable research in the last decade on lek-breeding birds, many aspects of the evolution of the lek social system remain unresolved. For example, it has been suggested that lek males aggregate because females prefer to visit groups of males rather than solitary individuals. Whether this is true or not, it addresses the issue of group maintenance rather than group origin. Originally, males had to aggregate independent of female preference so that females could then prefer the group. Initial male aggregations may have formed passively when males responded independently but concurrently to some external stimulus (e.g., a concentration of display sites, prominent food source, a travel corridor or nesting sites commonly used by females) that caused them to be drawn to a particular site at a certain time unmindful of other males. Aggregation was an unintentional by-product of that response. In contrast, male aggregation may have been active, with a male purposely seeking to associate with another male (or males) because proximity to that male allowed him to enhance his own reproductive success by improving his ability to attract females (i.e., learning from his associate) or to intercept females attracted to that male (i.e., poaching on his success). To identify factors important in the formation of male aggregations, I studied the Dwarf-tyrant Manakin (Tyranneutes stolzmanni), a species in which some breeding males occupy solitary courts where they attempt to attract females for reproduction, whereas other males occupy courts clustered into leks, where they interact to attract females. I compared habitat characteristics and resource availability at courts of these two groups as well as certain traits of males in each to gain insight into passive or active lek formation in this and the initial lek-forming species.