During June and July 2011, I audio-recorded eight Warbling Vireos (Vireo gilvus) at widely scattered sites in eastern Boulder County, Colorado, immediately east of the steep foothills of the Rocky Mountains. All eight sang songs like those of the eastern subspecies, V. g. gilvus; immediately to the west, in the steep foothills of the Rocky Mountains, Warbling Vireos sing songs like those of the western subspecies, V. g. swainsonii. The results of these observations suggest both the presence of nominate gilvus farther west in Colorado than previously documented and a sharp demarcation between the breeding ranges of swainsonii and gilvus.