Thick-billed fox sparrow
subspecies of bird
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)
Subspecies
The megarhyncha subspecies group breeds in the mountains from southern Oregon to southern California east to the Sierra Nevada and shows little geographic variation. It interbreeds with the slate-colored subspecies group along a narrow contact zone from southern Oregon to western Nevada (Rising & Beadle 1996) but as noted above, gene flow is quite limited.
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