The review by Zink & Weckstein (2003), which added mtDNA cytochrome "b", NADH dehydrogenase subunit 2 and 3, and D-loop sequence, confirmed the four "subspecies groups" of the fox sparrow that were outlined by the initial limited mtDNA haplotype comparison (Zink 1994). These should probably be recognized as separate species, but this was deferred for further analysis of hybridization. Particularly the contact zones between the slate-colored and thick-billed fox sparrows which are only weakly distinct morphologically were of interest; the other groups were found to be distinct far earlier. A further study of the nuclear genome, using microsatellites, showed similar separation between the four groups.