Rich in variation, Dark-eyed Juncos offer endless opportunities for study. The five subspecies groups (Slate-colored, Oregon, Gray-headed, White-winged, and Guadalupe) currently include 15 recognized subspecies (A.O.U. 1957, 1983), with the distinctive subspecies mearnsi (Pink-sided) plausibly considered a sixth group. Although it is easy to become overwhelmed by such complexity—the late Ernst Mayr (1942) termed the species a systematist’s “nightmare”—here we embrace only the modest goal of illustrating and reviewing one form of introgression, that between the Pink-sided Junco (J. h. mearnsi) and the Gray-headed Junco (J. h. caniceps). Junco h. mearnsi is generally considered part of the Oregon group, but in this discussion—purely for the sake of clarity and with no taxonomic implications—we treat it as a taxon separate and apart from the Oregon Junco, which consists of seven other subspecies.