Abstract
The longstanding problem of relationships among populations of bushtits in the southwestern United States and northern Mexico was pursued through field work and examination of specimens. Results for the Chisos Mountains of west Texas and other critical areas indicate a situation of polymorphism in head coloration, rather than 1 of 2 sympatric spp.; Psaltriparus melanotis and P. minimus are, therefore, conspecific. Black-eared individuals occur among juvenile and adult males and among juvenile females in various populations; the percentages of such individuals and the extent of black coloration in the individual increase from north to south in 2 clines, 1 centering in west Texas, the other in northeastern Sonora. Other color characteristics vary more or less concordantly. Ecology and behavior of black-eared and plain-eared forms appear identical. Patterns of variation suggest that clines are due to secondary inter gradation between 2 weakly differentiated groups, following a period of geographic isolation. Support for this interpretation is found in distributions of closely related forms in other genera of birds occupying similar habitats in the same region.

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