Thick-billed fox sparrow


The thick-billed fox sparrow 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, but according to mtDNA cytochrome b sequence and haplotype data, it forms a recognizable clade. Research on suspected hybridization and considering additional DNA sequence data led to confirmation of their distinctiveness ; this group appears to be most closely related to the sooty and/or slate-colored fox sparrows.
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'.

Subspecies

The megarhyncha complex breeds in mountains from southern Oregon to southern California east to the Sierra Nevada and shows little geographic variation. It interbreeds with the slate-colored complex along a narrow contact zone from southern Oregon to western Nevada but as noted above, gene flow is quite limited. Sibley indicates that this group has the most diagnostic call note, "a high, flat squeak teep like California towhee".