Systematics
The review of Zink & Weckstein (2003) which added mtDNA cytochrome b, NADH dehydrogenase subunit 2 and 3, and D-loop sequence confirmed the 4 "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 (Swarth 1920).
The combined molecular data is unable to resolve the interrelationship of the subspecies group and of subspecies in these, but aids in confirming the distinctness of the Thick-billed group (Zink & Weckstein 2003). Biogeography indicates that the coastal populations were probably isolated during an epoch of glaciation of the Rocky Mountains range, but this is also not very helpful in resolving the remaining problems of within-group diversity, and inter-group relationships.
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