Two recognized subspecies or allospecies, depending on taxonomic treatment (Am. Ornithol. Union 1957, Sibley and Monroe 1990, Doyle 1997): S. b. breweri Cassin, 1856 (Brewer’s Sparrow); and S. b. taverneri Swarth and Brooks, 1925 (Timberline Sparrow), which was originally described as separate species and is recognized as such by some authorities (Sibley and Monroe 1990). These forms differ in morphology, song, and breeding habitat (Swarth and Brooks 1925, Oberholser 1974, Pyle and Howell 1996, Doyle 1997). Breeding ranges are allopatric (see Distribution); winter range of Timberline Sparrow is poorly known, but so far as known lies within that outlined for species as a whole (see Distribution, above). Breeding habitat of nominate breweri is sage scrub or shrub-steppe; Timberline Sparrow occurs in subalpine-alpine transition with low canopy cover (Oberholser 1974, Doyle 1997).
In morphology, Timberline Sparrow differs from nominate breweri as follows: Coloration darker and grayer (versus browner), with stronger contrast between gray breast and white abdomen; dorsal streaking heavier and coarser, with bolder head pattern (including more distinctive supercilia), a greater tendency toward narrow streaking on breast and flanks, and more heavily streaked nape; larger size; and smaller, more slender, and more extensively dark bill. See Appendix for linear measurements indicating mean values, ranges in size, and overlap between the 2 subspecies. Some nominate breweri breeding in n. California are more heavily streaked, tending toward the Timberline (Grinnell et al. 1930), although this claim is contentious. Nominate breweri breeding along western edge of their range are redder and show more distinct median crown-stripe than do birds farther east (Rea 1983: 237). Field identification of Timberline Sparrow is problematic; more work is needed.
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Songs of breweri and geographically distinct taverneri races are superficially similar. However, trills of breweri have wider frequency range and hence sound more "buzzy." Playback of taverneri song to taverneri birds produced a more aggressive response than did playback of breweri song (Doyle 1997).
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Known breeding range of Timberline Sparrow (S. b. taverneri) includes 2 widely separated areas. Breeds in mountains of extreme sw. Alberta (Swan 1946, Sememchuk 1992) and probably in adjacent mountains of extreme se. British Columbia (Campbell et al. in press). Also breeds from extreme nw. British Columbia (from Atlin area west; Campbell et al. in press) northwest through sw. Yukon (Godfrey 1986). Small population in se. Alaska Range (Nutzotin Mtns.) of Alaska (Doyle 1997). Likely breeds in suitable habitat in mountains between known areas from sw. Alberta to sw. Yukon, but few surveys in this remote region.
