The thick-billed fox sparrow (Passerella (iliaca) megarhyncha) group comprises the peculiarly large-billed
Sierra Nevadantaxa in the
genusPasserella. It is currently classified as a "subspecies group"[1] 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
mtDNAcytochrome bsequence 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.[2] 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. Sibley (2000) indicates that this group has the most diagnostic call note, "a high, flat squeak [sic] teep like
California towhee".
megarhyncha Baird, 1858:
Breeds from the
Onion Mountains and
Robinson's Butte (southwestern Oregon) south through inland northern California to
Kearsarge Pass (
Inyo County), as well as the western flank of the Sierra Nevada in
Mono County. Winters in central and southern California and adjacent
Mexico.(Weckstein et al. 2002)
More larger-billed, duller, and grayer than the schistacea slate-colored fox sparrows; intermediate between the very large long-tailed stephensi and the more schistacea-like monoensis of this group; intergrades with the former.(Weckstein et al. 2002)
The largest, most massive-billed and longest-tailed fox sparrow; also very gray in coloration. (Swarth, 1920) Intergrades with megarhyncha. (Weckstein et al. 2002)
Breeds on the eastern flank of the Sierra Nevada in Mono County, California, and in the
Walker River Range in adjacent
Mineral County, Nevada. Winters from inland central California coastwards and south into Mexico. (Weckstein et al. 2002)
Somewhat intermediate between megarhyncha and schistacea slate-colored fox sparrows, with a paler, ashy gray back. (Weckstein et al. 2002)
A browner version of monoensis; in winter range, distinguished from olivacea slate-colored fox sparrows by decidedly ruddy brown and larger bill. (Weckstein et al. 2002)
mariposae Swarth, 1918:
Breeding in a limited area in California between the
headwaters of the
Little Shasta River (
Siskiyou County) to
Yosemite National Park and Kearsarge Pass. Winter range undocumented (Weckstein et al., 2002) due to lack of distinctiveness but probably same as megarhyncha and monoensis.
Probably nothing more than an intergrade between megarhyncha and monoensis; nowadays usually
synonymized with the former. (Rising & Beadle 1996)
^Etymology: megarhyncha, "large-billed", from
Ancient Greekmega-, "great" + rhynchos, "beak".
Rising, Jim D. & Beadle, David (1996): A Guide to the Identification and Natural History of the Sparrows of the United States and Canada. Academic Press, San Diego.
ISBN0-12-588971-2
Swarth H. W. (1920): Revision of the avian genus Passerella with special reference to the distribution and migration of the races in California. University of California Publications in Zoology21: 75–224.
Weckstein, J. D.; Kroodsma, D. E. & Faucett, R. C. (2002): Fox Sparrow (Passerella iliaca). In: Poole, A. & Gill, F. (eds.): The Birds of North America715.
Academy of Natural Sciences, Philadelphia, PA &
American Ornithologists' Union, Washington, D.C. Online version, retrieved 2006-11-27.
doi:
10.2173/bna.715 (requires subscription)
Zink, R. M. (1994): The Geography of Mitochondrial DNA Variation, Population Structure, hybridization, and Species Limits in the Fox Sparrow (Passerella iliaca). Evolution48(1): 96-111.
doi:
10.2307/2410006 (HTML abstract, first page image)
Zink, Robert M. & Kessen, A. E. (1999): Species Limits in the Fox Sparrow. Birding31: 508-517.
Zink, Robert M. & Weckstein, Jason D. (2003): Recent evolutionary history of the Fox Sparrows (Genus: Passerella). Auk120(2): 522–527. [Article in English with Spanish abstract]
DOI: 10.1642/0004-8038(2003)120[0522:REHOTF]2.0.CO;2
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