Hybrid zones are often described as clines of genetic and phenotypic traits moving across species barriers through introgression. Yet, hybrid zones can also be spatially complex and shift over time, and dispersal and vicariance can isolate portions of a cline, potentially leading to hybrid lineage formation. We reassessed Sibley’s (1950) gradient between Collared Towhee (Pipilo ocai) and Spotted Towhee (P. maculatus) in Central Mexico to test whether it conformed to a typical tension-zone cline model....