This individual represents the first record for Northern California. It was first reported on eBird on 8 January by Kelly Childress.
It shows characters suggesting a member of the curvirostre Group which includes T. c. celsum, T. c. oberholseri, and T. c. curvirostre. Prior California records have been attributed to the palmeri Group characterized by lack of wing-bars, smaller tail spots, darker underparts, and less contrasting ventral spotting.
In addition this bird shows the breast spots clustering in the middle instead of being spread uniformly. Note also the sharp contrast between the white throat and the breast. In the palmeri group this transition is more blended. The tail spots are bright white on our bird. In the palmeri group they are grayish-white and much less distinct.
This individual could reasonably be assigned provisionally to T. c. celsum which has spread as far north as southern Colorado and Kansas. It is described as more buff on its underparts with whiter tail tips cf. nominate. However, differences are slight. These races intergrade and some authors consider celsum and oberholseri to be synonyms of curvirostre. For these reasons it would probably be unwise to assign this bird to any particular subspecies within the curvirostre Group.
Digiscoped with Panasonic DMC-LX5 | Nikon FieldScope III | 30XWA | hand-held (no adapter).
References:
Cody, M. (2018). Curve-billed Thrasher (Toxostoma curvirostre). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/58207 on 16 January 2018)
Leukering, T. and Pieplow, N. 2014. Identifying Curve-billed Thrasher Subspecies: Why and How. Colorado Birds 48:296-304.
Phillips, A. R., Jr. Marshall, J. T. and G. B. Monson. 1964. The birds of Arizona. Tucson: Univ. of Arizona Press.
Phillips, A. R. 1986. The known birds of North and Middle America, Part I. Hirundinidae to Mimidae; Certhiidae. Denver, CO: Denver Mus. Nat. Hist.
Pieplow, N. 2010. Curve-billed Thrasher Identification. Blog post: http://earbirding.com/blog/archives/2480
Pyle, P. 1997. Identification Guide to North American Birds, Part I Columbidae to Ploceidae. Slate Creek Press.
Sibley, D. A. 2010. Curve-billed Thrasher - Sibley Guides. Blog post: http://www.sibleyguides.com/bird-info/curve-billed-thrasher/
Tweit, Robert C. 1996. Curve-billed Thrasher (Toxostoma curvirostre), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA. https://doi.org/10.2173/bna.235