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Hermit Warblers (Dendroica occidentalis) sing distinct dialects of type I songs, the most common song before pairing. Eight dialects were identified and described in a 22 900 km² area in southwestern Oregon and northern California. The dialects were well defined geographically with contact areas between dialects seldom extending more than 6 km. Gaps in forested habitat of ≥10 km separated several dialects, but within forested areas dialect boundaries did not conform to obvious habitat, elevation, or geographic boundaries. Few songs containing syllables or phrases from more than one dialect were identified, and birds incorporating elements from two different dialects inhabited areas close to the common boundary between the two. Multivariate analysis showed that birds in neighboring areas had dialects most similar in structure, but a more complex history of dialect development or origin is suggested in other areas. [ABSTRACT FROM AUTHOR]