Knowledge of the migration geography of Flammulated Owls (Otus flammeolus) is limited. We combined data from multiple studies in the western United States to evaluate patterns of variation in Flammulated Owl morphometrics. Measurements from autumn migration study sites in New Mexico, Nevada, and Idaho followed a geographic cline from southeast to northwest across the species' range. Our objectives were to: (1) describe age- and sex-specific variation in the morphometrics of Flammulated Owls captured at the three sites, and (2) discuss the implications of this variation for understanding the species' migration geography. Based on a hierarchical series of factorial two-way ANOVAs, we discovered significant overall sex-specific differences in wing chord, exposed culmen length, and hallux length, and a consistent pattern of site-specific differences for most measurements. Most measurements increased from southeast to northwest, consistent with previous data on clinal variation in the species. Evidence of significant variation in the morphometrics of owls captured at the three migration sites suggests little longitudinal mixing of migrants in the region.