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STERN TANAGER^ Los Angeles Audubon Society Volumes£^\ Number^ *f December 1984 The Identification of Curve-Billed, Bend ire's and Gray Thrashers By Jon L. Dunn and Kimball L. Garrett he family Mimidae, now thought to be closely allied to the starlings, Tperhaps at the family level (see Sibley andAhlquist, Auk 101:230-243,1984), includes ten species of thrashers of the genus Ioxostoma. These thrashers are all slender, predominantly brown or gray-brown birds with long, slender and often strongly decurved bills; they inhabit brushy country, often quite arid. The ten species appear to fit into three species complexes. One com- plex includes the Brown Thrasher (I. rufum), the Long-billed Thrasher (I. longirostre) and the endemic thrasher of Cozumel Island off the Yucatan Peninsula, I guttatum. Another complex includes the Crissal, California and Le Conte's Thrashers. These three species are primarily ground dwellers in brushy habitats; they are excellent runners. The Crissal (I. crissale)1 and California (I. redivi- vum) Thrashers are closely related and essentially allopatric (but beware near over- lap along the western edges of the Califor- nia deserts, such as in the upper Coachella 1 Called Toxostoma dorsale in most recent publications, including the sixth edition of the A.O.U. Check-List; a very recent decision by the International Commission on Zoological Nomenclature suppresses dorsal and mandates the adoption of crissale (see Auk 101:348, 1984). Artist: Jonathan Alderfer Santa Monica Top to bottom: Toxostoma curvirostre — Curved-billed Thrasher T.C. curvirostre, adult T. C. palmeri, adult T.C. palmeri, immature T. bendirei — Bendire's Thrasher T. — Gray Thrasher Valley and the western portion of the Bor- an desert regions of southern Arizona; most plumage. Also beware that, as thrashers rego Valley). The Le Conte's Thrasher (I. individuals of the species winter in north- have only one annual molt (in fall), there is lecontei) overlaps extensively with Crissal western Mexico. While long-distance vagran- a pronounced difference between fresh- and very locally with California, but prefers cy is virtually unknown in the Bendire's plumaged birds in fall and winter and worn a more arid, open habitat. These three spe- Thrasher, observers east and north of the birds in spring and summer. This may affect cies are almost totally sedentary, and an species' range should be alert for its the ventral spotting patterns described below understanding of distinction and habitat occurrence. to the point where such patterns are of little preferences should minimize identification The Curve-billed Thrasher is resident from or no use in identification. Bendire's in par- problems. central Arizona, central New Mexco, the ticular may appear essentially unspotted by The final group of thrashers is the subject extreme southwestern Great Plains, and spring and, especially, summer. of the present article. The Curve-billed south-central Texas south through central The overall plumage color of Bendire's is Thrasher (I. curvirostre) shows important Mexico. It breeds on the Sonoran Desert tannish brown, differing from the somewhat geographical variation, unlike most of the westward virtually to the Colorado River, darker gray-brown color of the Curve-billed. other species of thrashers. Though not highly but remains a strictly casual visitor to Cali- Both species have a whitish throat, but the migratory, it is given to wandering, and fornia. California records come from the throat of the Curve-billed appears a little vagrants have been recorded far out of Colorado River and Imperial Valleys, prim- more conspicuously set off from the gray- range. The closely related Ocellated Thrasher arily in winter (there are no valid winter brown chest and the dark cheek stripe (the (I. ocellatum) of the southern Mexican plat- records before late October). Unlike the white throat does not stand out as much on eau will not be discussed here. The Ben- Bendire's Thrasher, there are at present no the paler-breasted Bendire's). The color of dire's Thrasher (I. bendirei) differs from our proven records for the coast. Claimed Curve- the Bendire's is richest on the flanks and other thrashers (apart from the Brown billeds from west and north of the Imperial vent (though this color maybe obsolete on Thrasher) in being distinctly migratory; as Valley should clearly be substantiated with worn birds); Curve-billeds also show a ten- such, out-of-range vagrants are noted with detailed descriptions and, ideally, photo- dency toward cinnamon-buff in the vent some frequency. Closely related to the graphs. area. Bendire's is the Gray Thrasher (I. cinereum), The Curve-billed Thrasher is a larger, The breast and sides of the Bendire's are a species endemic to the Baja California bigger-headed, less compact bird than Ben- marked with short, fine inverted "V" marks, peninsula. Because the Gray Thrasher occurs dire's, and in this regard it suggests the loosely arranged in rows. In contrast, the within about 100 miles of the California shape shown by the California/Crissal com- Curve-billed shows larger, blotchier spots border, and because the Bendire's Thrasher plex. This similarity is heightened by the which do not form a distinct pattern. It must occasionally occur as a vagrant within long, conspicuously decurved bill. The much should be noted here that the eastern and its range, we have treated the species briefly shorter, straighter, and proportionately southern races of Curve-billed Thrasher in this article. (For a discussion of the prob- slightly thinner bill of the Bendire's is gen- (typified by nominate curvirostre) have able evolutionary history of the different erally an excellent field character. In the larger, more noticeable ventral spots on a Toxostoma species complexes, see Hubbard, Bendire's there is no decurvature to the paler background (see plate). The Sonoran J.P., "Avian evolution in the aridlands of lower mandible, in contrast to the definite Desert birds, palmeri, have less distinct ven- North America", The Living Bird 12:155-196, lower mandible decurvature shown by the tral spots on a darker background. The Chi- 1973). Curve-billed. Observers should be aware ricahua Mountains of southeastern Arizona Because both Bendire's and Curve-billed that juvenile Curve-billeds, in their first form the dividing line between these two Thrashers are prone to wandering well out month or so out of the nest, have bills racial groups, although there is certainly of range, observers should be familiar with which may resemble those of Bendire's some intergradation in this area (Chirica- the characters which distinguish them. Field closely; these confusing individuals are not hua birds are closer to palmeri in appear- guides have traditionally emphasized bill likely to be seen away from the breeding ance); for more detail, see Phillips, A.R., et shape and eye color as important distin- grounds, but they may not be accompanied al, Birds of Arizona, 1964. Beware that many guishing features. Below we will caution by the more easily identified adults, and adult Curve-billeds do show smaller inverted that the former character can be confusing sometimes co-occur with Bendire's. Even "V" marks, although these are concentrated when juvenile Curve-billeds are considered, short-.billed Juvenile Curve-billeds show a toward the center of the lower breast, below and that eye color is of little or no use as an pronounced gonydeal angle on the lower the larger, more diffuse spots of the upper identifying character. We suggest additional, mandible, in contrast to the straight or very breast and sides. On adult Bendire's the more reliable characters. slightly angled mandible of the Bendire's. smallest spots are at the top of the center of The Bendire's Thrasher breeds locally Bill color characters (see below) appear to the chest, just below the throat. Keep in from southern Nevada and southern Utah be a diagnostic after the young Curve-billeds mind that juveniles of both species show south to southern Sonora. In California it is have lost the last traces of the pale flanges at fine, narrow breast streaking, and that the a locaJ breeder on the Mojave Desert, par- the gape. breast markings of worn adults (even by ticularly in Joshua Tree/juniper woodlands Bill color is consistently different between mid-spring) may be obsolete. As a further in eastern San Bernardino County. A very Bendire's and Curve-billed Thrashers and is footnote, we point out that extremely worn few breed south to the Yucca Valley and thus a critical indentifying character. The adult Sage Thrashers (oreoscoptes monta- Joshua Tree National Monument areas, and bill of the Curve-billed is entirely dark nus) in late summer may essentially lack singing birds have been recorded in spring (nearly blackish). That of the-Bendire's is apparent streaking below and superficially in the Antelope Valley. Transients are noted slightly paler (grayer) throughout and, most resemble the larger, longer-billed Bendire's only very rarely away from the breeding importantly, always shows some pale fleshy, Thrasher. grounds, but vagrants have been noted in fleshy-yellow, or horn color at the base of All members of the Bendire's/Curve-billed the interior as far north as Mono County. the lower mandible. While diagnostic, this Thrasher complex show some degree of Vagrant Bendire's Thrashers appear on the mark does require careful scrutiny in the paleness on the corners of the tail. Again coastal slope very rarely but regularly, pri- field. this is most pronounced in fresh birds, and marily in September and October. The spe- In discussing plumage characters separat- the character may become obsolete by mid- cies is casual in winter in California, with ing Bendire's and Curve-billed Thrashers, it to late spring.

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