In a comprehensive investigation of macrogeographic variation in the Plain Titmouse (Parus inornatus) complex in western North America, the author assessed population-level patterns of differentiation in morphometric, colorimetric, allozymic, mtDNA, and vocal characters. These suites of traits showed broad geographic concordance, distinguishing Pacific slope from interior populations. These two groups of populations are treated as sibling species. University of California Publications in Zoology Volume 128. Black and white photos and maps. Natural history and variation.