The dark-eyed junco is a single species containing strongly differentiated regional forms. A slate-coloured bird from eastern Canada is uniformly dark grey; an Oregon junco from the west has a black hood contrasting with a brown back and pink flanks; a grey-headed junco from the southern Rockies is pale with a rufous back patch. Each was described as a distinct species before the twentieth-century revisions merged them.
Genetic work supports the merger and adds a striking detail: the diversification of these forms is very recent, on the order of the last glacial retreat, making juncos a favoured system for studying rapid divergence. Where forms meet, they interbreed, which is why they are ranked as subspecies groups rather than species.
A more recent case runs the other way. A junco population established in San Diego since the early 1980s has diverged measurably in behaviour and in the amount of white in the tail, showing that measurable change in this system can occur inside a few decades.