We use the natural propensity for least flycatchers (Empidonax minimus) to form dense territorial clusters to test the hidden lek hypothesis as an explanation for clustering behavior. The hidden lek hypothesis proposes that socially monogamous males can cluster their all-purpose territories owing to female pursuit of extrapair copulations, analogous to females seeking promiscuous copulations at leks. We define a hidden lek as a cluster of all-purpose territories that resembles a classical lek but whose characteristics are less overt because of larger territory sizes, a pair bond between the territorial male and female, and biparental care. We tested four predictions: (1) clustered males should be preferred by females as social mates; (2) late-arriving males in clusters should settle next to early-arriving males; (3) males near central males should pair before peripheral males; and (4) spatial position (centrality) should explain much of the variation in male social and genetic mating success. Females preferred to pair with clustered males. In clusters, compared with peripheral males, central males arrived earlier, were heavier and in better body condition, and had higher pairing success. Microsatellite profiling from 1999 and 2000 revealed that some extrapair paternity was more common in peripheral nests, but we did not detect a skew in genetic mating success favoring central males. We conclude that least flycatcher breeding clusters resemble hidden leks in some predicted ways, but our data are insufficient to conclude that female mating behavior promotes clustering. We discuss alternative hypotheses for clustering and additional possible ways to test the hidden lek hypothesis in this species. This is the first test of the hidden lek hypothesis in a socially monogamous, all-purpose territorial bird.