The Plain Titmouse (Parus inornatus) varies in overall size, bill size, and color throughout its range in western North America. Ten geographic forms are currently recognized (American Ornithologists' Union, 1957, Check-list of North American Birds) on the basis of morphology and color. West of the Cascade-Sierra Nevada Mountains, the species occurs commonly in foothill oak woodland. Density is lower in the Southwest and Intermountain Regions, where populations are found patchily in pinyon-juniper or juniper habitat. The goal of this project is to integrate morphologic, molecular, and behavioral data on Parus inornatus in order to elucidate patterns and processes of divergence among populations. Differences in population structure will enable testing of alternative models of gene flow. Hypotheses concerning the role of adaptation and Pleistocene isolation also will be evaluated. A variety of modern systematic techniques will be used to examine patterns of morphologic and genetic variation within and among populations of Parus inornatus. Results to date indicate strong genetic differentiation between Pacific slope and Intermountain populations, on the order of that found between closely related species of birds. Patterns of variation in morphology (especially bill size and shape) and color support the genetic data. A quantitative estimate of gene flow will shed light on the process of genetic and morphologic divergence among populations. Because numerous bird species show similar patterns of divergence between the Pacific slope and Intermountain Regions, this study will contribute to an emerging picture of avian speciation in western North America. The results of this study will likely lead to formal taxonomic changes by the American Ornithologists' Union Committee on Classification and Nomenclature. The identification of genetically and/or morphologically distinct populations will provide critical information for managers concerned with the protection and management of pinyon-juniper or oak woodlands in the western United States. The future of oak woodlands in California is of growing concern because of increasing pressures for development in the foothills of the Cascade-Sierra, Nevada and Coast Ranges.
