Although first-category (Type I) songs of some wood-warblers (Parulidae) are structurally stable over time and range, we document a rapid change in the song of a population of Hermit Warblers (Setophaga occidentalis) in northern Oregon. Type I songs on Mount Hood were recorded in 1990 and 1992. When the population was revisited in 2003, the birds in the area sang a different Type I song. The distinctive terminal syllable was absent, the introductory series of repeated syllables had differentiated into two parts, and the first phrase had increased in duration. In 2004, an area within a 20-km radius centered on the original population was systematically searched; from 2004 to 2006, selected areas were searched up to 65 km distant. Five scattered individuals in a limited area were found singing the earlier song 23–28 km to the east, at the periphery of the area dominated by the new song. The differences between the earlier and later songs, which are smaller than differences with neighboring dialects, suggest that the song changed within the current population rather than via displacement by individuals from an adjacent song population. Structural change over time in a Type I song of a wood-warbler over an area larger than a neighborhood has not been previously documented.
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Vol. 130 • No. 1