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).
... Beadle, David (1996) A Guide to the Identification and Natural History of the Sparrows of the United States and Canada ... (2002) Fox Sparrow (Passerella iliaca) ... Structure, hybridization, and Species Limits in the Fox Sparrow (Passerella iliaca) ...
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