 It has long been suspected to be a separate evolutionary lineage due to morphological distinctness (Swarth 1920), and this is confirmed by analysis of mtDNA sequence and haplotype data (Zink 1994, Zink & Kessen 1999, Zink & Weckstein 2003).This group appears to be most closely related to the thick-billed and/or slate-colored fox sparrows (Zink 1996, Zink & Weckstein 2003).The sooty fox sparrow complex varies clinally in intensity of color.