Bell’s Vireo (Vireo bellii) has four recognized subspecies: V. b. bellii, which breeds from eastern Colorado east to the Mississippi River and south to central Texas and Tamaulipas; V. b. medius, which breeds from southwestern Texas south to Durango and Coahuila; V. b. arizonae, which breeds from the Colorado River east through Arizona and south to Sonora; and V. b. pusillus, the Least Bell’s Vireo, which breeds in California and northwestern Baja California (AOU 1957, Brown 1993, Unitt 1985, Pyle 1997).