Habitat characteristics and nest predation do not explain clustered breeding in least flycatchers (Empidonax minimus)

Animals often exhibit territorial spatial structure in their breeding habitat. This clustering behavior is not well understood. We reviewed eight hypotheses for clustering and tested two ecological hypotheses for the formation of dense, territorial clusters in the Least Flycatcher (Empidonax minimus), a socially monogamous forest bird. The material resources hypothesis suggests that clustering is a response to habitat heterogeneity in vegetation, food, or both. The predation hypothesis proposes that clustering may reduce nest predation. Univariate and multivariate analyses of 170 vegetation plots from 1997 to 1998 indicated that forest-stand structure and tree species composition could not explain clustering in our population (predictions 1–3). Comparison of mean arthropod biomass inside with arthropod biomass outside two clusters sampled in 1999 using Malaise traps revealed that potential food resources were also unrelated to clustering (prediction 4). Nest predation rates were not correlated with territory position in clusters or with cluster size. In addition, predation rates were similar for clustered and solitary pairs (predictions 5–7). We conclude that habitat characteristics and nest predation do not explain clustered breeding in Least Flycatchers, though further tests of those hypotheses would be helpful. We develop the idea that the pursuit of extrapair copulations may promote clustered breeding. Future studies of territorial spatial structure in Least Flycatchers and other species should consider explanations based on mating behavior concomitant with ecological explanations for clustering.