The Thick-billed Fox Sparrow (Passerella (iliaca) megarhyncha) group comprises the peculiarly large-billed Sierra Nevadan taxa in the genus Passerella. It is currently classified as a "subspecies group" within the Fox Sparrow pending wider-spread acceptance of its species status.
These birds were long considered members of the Slate-colored Fox Sparrow group due to morphological characteristics (Swarth 1920), but according to mtDNA cytochrome b sequence and haplotype data (Zink 1994), it forms a recognizable clade. Research on suspected (Rising & Beadle 1996) hybridization and considering additional DNA sequence data led to confirmation of their distinctiveness (Zink & Kessen 1999); this group appears to be most closely related to the Sooty and/or Slate-colored Fox Sparrows (Zink 1996, Zink & Weckstein 2003).
Thick-billed Fox Sparrows are almost identical in plumage to Slate-colored Fox Sparrows but have a more extensive blue-gray hood and a less rusty tail. The most striking feature of this bird is its enormous beak which can appear to be three times as large as that of the markedly small-billed Slate-colored Fox Sparrows A Thick-billed Fox Sparrow's beak also differs in color from that of the Slate-colored. Although the culmens of both groups are grayish brown, Slate-coloreds have yellow lower mandibles instead of the steel blue of the Thick-billeds' (Rising & Beadle 1996).
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