Generally speaking, Red-shafted Northern Flickers are found in western North American and Yellow-shafted Northern Flickers are found in eastern North America. It should be noted, however, that where the ranges of each overlap—a relatively narrow swath that runs from southern Alaska to Northern Texas—they interbreed, creating hybrids with shafts varying in color between the bright red and the bright yellow. The males mustaches also vary between the black and the bright red in instances of interbreeding.
I consider myself quite lucky to have a pair of Yellow-shafted Northern Flickers frequently visiting the feeders of my old Kentucky home.
I'm hoping to see their offspring at my feeders in the not too distant future.