A phenomenon currently not well understood is the reproduction of some species in clustered territories. We evaluated two ecological hypotheses (material-resources and predation hypotheses) that could explain cluster formation during reproduction of the Blue-black Grassquit (Volatinia jacarina), a socially monogamous bird with a high rate of extra-pair fertilization. Additionally, we considered the breeding synchrony of females, since synchronization may serve as a means of diluting predation. To this end, we compared the breeding synchrony of females in clusters to that of those in solitary territories. We found support for the material-resources hypothesis in terms of vegetation structure and food availability: territories were clustered in habitat with more food and a more complex vegetation structure. Breeding synchrony of females in clusters was higher than that of females in solitary territories, but probably not as a mechanism to dilute predation: pairs within clusters suffered lower reproductive success because of elevated predation. The advantages gained by reproducing in clusters in areas of more complex vegetation structure and more food may offset the disadvantages due to predation, especially over a longer temporal scale. Our results suggest that the aggregation of Blue-black Grassquit territories could be explained by habitat heterogeneity in terms of vegetation and food, but these results do not exclude other hypotheses related to sexual selection and/or competition leading to aggregation.