A persistent challenge in ecology is to tease apart the in-fluence of multiple processes acting simultaneously and interacting in complex ways to shape the structure of species assemblages. We implement a heuristic approach that relies on explicitly defining spe-cies pools and permits assessment of the relative influence of themain processes thought to shape assemblage structure: environmental fil-tering, dispersal limitations, and biotic interactions. We illustrate our approach using data on the assemblage composition and geographic distribution of hummingbirds, a comprehensive phylogeny and mor-phological traits. The implementation of several process-based species pool definitions in null models suggests that temperature-but not pre-cipitation or dispersal limitation-acts as the main regional filter of as-semblage structure. Incorporating this environmental filter directly into the definition of assemblage-specific species pools revealed an other-wise hidden pattern of phylogenetic evenness, indicating that biotic in-teractions might further influence hummingbird assemblage structure. Such hidden patterns of assemblage structure call for a reexamination of a multitude of phylogenetic-and trait-based studies that did not ex-plicitly consider potentially important processes in their definition of the species pool. Our heuristic approach provides a transparent way to explore patterns and refine interpretations of the underlying causes of assemblage structure.