posted October 19th, 2010; last edited November 18th, 2010 –– David Sibley

Curve-billed Thrasher Toxostoma curvirostre

Geographic Variation

  • Arizona (Sonoran, or Western) – T. c. palmeri group1
  • Texas (Chihuahuan, or Eastern) – T. c. curvirostre group2

These two populations are labeled “Arizona” and “Texas” in the Sibley Guide, but here I’ve chosen to use the more accurate regional names Sonoran and Chihuahuan, respectively.

Distribution and contact zone:

Range of two populations of Curve-billed Thrasher. Sonoran population shown in red, Chihuahuan in blue, and area of intergradation in green. Red markers show vagrant records of Sonoran. Records north and east of mapped range (not shown; to Manitoba, Minnesota, Wisconsin, etc.) may be all Chihuahuan. A few are confirmed as that subspecies, and this suggests that Chihuahuan birds could occur west of the mapped range as well.

Generally, Sonoran birds are found in Arizona and Chihuahuan birds east of there in New Mexico and Texas. Phillips et al3 report that the only true Chihuahuan Curve-billed Thrashers in Arizona are found in the extreme southeast corner – the east slope of the Chiricahuas and in Guadalupe Canyon. From there west to Patagonia and Tucson they report a gradual shift to Sonoran-like features, with more Chihuahuan-like birds at higher elevations and Sonoran-like birds in the valleys.

The presence of Chihuahuan-like birds as far west as Nogales4 and Tucson, Arizona5 might represent incursions by birds from eastern populations, or variability within the local population. There is no comparable information for New Mexico, but presumably a similar transition occurs to the east across New Mexico, with intermediate birds and a gradual shift to more Chihuahuan-like features (more study badly needed). Within the Chihuahuan group there seems to be a cline of paler, grayer, and plainer birds in the west (New Mexico) to darker and browner birds with more contrasting markings in the east (southern Texas).6

Zink and Blackwell-Rago (2000) found distinct differences in mitochondrial DNA more or less coinciding with the differences in plumage, but one sample collected near Albuquerque, New Mexico matched the Sonoran group. Rojas-Soto (2003) studied differences in breast spots and tail spots as well as measurements, and recommended that these two forms should be recognized as separate species. Birders can add significantly to the understanding of this complex by carefully observing plumage variation across Arizona and New Mexico.

Subspecies Identification – Sonoran vs. Chihuahuan:

Note 1: the following applies to adults in relatively fresh plumage October through May. During the summer some very worn individuals will require more care in identification, but the same criteria should work. Juveniles (March to October) have different plumage and cannot be identified by these criteria. They quickly become worn, and then begin molting to adult-like plumage, incoming feathers might provide the clues necessary for ID.

Note 2: As with any subspecies identification, no single field mark will reliably distinguish these two forms. Accurate identification depends on checking all of the field marks and building a case for the likely identification.

  • Spots on underparts more or less uniformly distributed on Sonoran (vs. clustered on breast and forming the suggestion of a dark breastband)
The lack of a breastband of spots on Sonoran birds may be due to the fact that the paler spots simply disappear against the slightly darker gray ground color, but the impression of a dark breastband of spots is a fairly obvious and consistent feature of Chihuahuan birds. This band is often especially strong across the lower breast, where it forms an even stronger contrast to the pale belly.
  • White throat blends smoothly to pale gray breast on Sonoran (vs. small dark spots encroaching from breast onto edges of white throat)
On Sonoran birds the pale gray of the breast blends smoothly into the dingy white throat, and darker spots are faintly visible, if at all. On Chihuahuan birds the more distinct dark breast spots continue forward onto the edges of the white throat, where they are clearly visible and obscure the smooth transition of the ground color from gray breast to white throat.
  • Tail spots grayish-white and indistinct on Sonoran (vs. more obvious bright white spots with clean and distinct edges)
Both forms have pale tips on all but the central pair of tail feathers. There is no difference in the extent or pattern of these pale spots. In Sonoran birds the tips may appear just slightly paler or nearly white, and blend gradually into the gray of the feather. On Chihuahuan birds the tail feathers are darker gray than on Sonoran and are tipped with bright white spots that contrast abruptly with the rest of the feather.
  • Wingbars faint or absent on Sonoran (vs. wingbars bright white and sharply contrasting)
Both forms have narrow pale tips on the greater and median coverts. The difference is in the degree of contrast and can be difficult to judge. Also – as with the tail spots – Chihuahuan birds show a combination of darker feathers and brighter white tips, and therefore greater contrast. Sonoran birds have a paler gray ground color on the coverts, and dingy gray-buff tips, and so, usually, show very little contrast.
  • Wings overall paler, contrasting little with the underparts (vs. darker, contrasting slightly but distinctly with the underparts)
Sonoran birds appear more or less uniform in color overall, with wings about the same color as the flanks. Chihuahuan birds appear slightly but distinctly two-toned, with dark upperparts and paler underparts. This is subtle but seems consistent and might be especially useful at a distance.
  • Tail feathers paler gray, especially outermost (vs. darker, blackish-gray)
This is fairly obvious when comparing specimens side-by-side, but would be difficult to see and to judge in the field. It contributes to the more contrasting white tail spots of Chihuahuan, and the paler outermost feather on Sonoran might be visible on a bird flying away.
  • Sonoran slightly paler and more grayish overall (vs. darker and more brownish overall)
  • Spots on breast usually less distinct (vs. more distinct)
More distinct spots on the Chihuahuan birds are the result of both darker spots and paler ground color.
  • Flanks and vent darker gray (vs. paler gray)
This is variable and difficult to judge. It is a contributing factor in the overall monochromatic appearance of Sonoran and two-toned appearance of Chihuahuan.
  • Belly pale gray, slightly paler than flanks and breast but usually not whitish (vs. belly usually whitish, contrasting strongly with the dark-spotted breast and somewhat with the pale gray flanks)
  • Calls of Sonoran birds are more like wit-WEET with second syllable louder and higher than first (vs. Chihuahuan call WIT-WIT with both syllables the same
Thanks to Nathan Pieplow for posting about this at http://earbirding.com/blog/archives/2480 . Both populations can give either two- or three-syllabled calls (although it has been suggested that Chihuahuan give mostly three-syllabled and Snoran mostly two-syllabled calls). More study is needed to establish ranges of variation (Nathan Pieplow discusses some odd calls from within the range of Sonoran). It has also been suggested that Chihuahuan birds may use mimicry in their songs more often than Sonoran birds, and there may be other subtle differences in songs. A detailed comparison of vocalizations is needed.

Unreliable or untested features:

  • Small differences in structure have been reported
A study by Rojas-Soto (1998, mentioned by Omland 2001) found small but consistent differences in size and shape between Sonoran and Chihuahuan groups. This is unlikely to be very helpful in the field, but is worth looking for.

References:

Omland, K. E. 2001. Thrashing Out Species Limits in the Southwest: A Curve-billed Examination. Birding 33: 320-328.

Rojas-Soto, O. R. 2003. Geographic variation of the Curve-billed Thrasher (Toxostoma curvirostre) complex. Auk 120: 311-322.

Tweit, Robert C. 1996. Curve-billed Thrasher (Toxostoma curvirostre), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/235

Zink, R. M. and R. C. Blackwell-Rago. 2000. Species Limits and Recent Population History in the Curve-billed Thrasher. Condor 102:881-886. pdf here: http://elibrary.unm.edu/sora/Condor/files/issues/v102n04/p0881-p0886.pdf

Notes:

  1. includes subspecies palmeri, maculatum, and occidentale, which are not widely recognized []
  2. includes subspecies curvirostre, celsum, and oberholseri, which are no longer widely recognized []
  3. 1964. The Birds of Arizona []
  4. specimen at Museum of Comparative Zoology, Harvard University []
  5. Rick Wright listserv message []
  6. specimens examined at Yale Peabody Museum []

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