We examined the phylogeography of the Western Hemispheric Sedge Wren (Cistothorus platensis) which occurs in grasslands from Canada to Tierra del Fuego. Genetic data indicate that Sedge Wren is paraphyletic with Mérida (Cistothorus meridae) and Apolinar's (Cistothorus apolinari) wrens and the currently recognized Sedge Wren is composed of a minimum of eight species. Speciation within this complex appears to have accelerated with the formation of the Isthmus of Panama which enabled the ancestor to occupy and differentiate in grassland habitats from sea level to above tree line in the Andes on the South American continent. As a result of the dramatic, negative anthropogenic impact on grasslands across the entire Western Hemisphere all species have suffered substantial declines and several are now threatened.