Temporal stability in the acoustic structure of dialect songs is a critical variable for studies of cultural evolution in songbirds. Here we report that the structure of the flight whistle song of Brown-headed Cowbirds (Molothrus ater) of the Convict dialect in the eastern Sierra Nevada of California remained essentially stable over almost 30 years. Quantitative measurements showed a minor structural change in one of four syllables and a small but statistically significant increase in duration for whistles recorded in 2006 versus 1990. Audiospectrograms of whistles recorded in the late 1970s revealed no major structural differences from the 1990 and 2006 whistles. These results are consistent with field and ontogenetic studies showing that young cowbirds copy the songs of adult males with very high fidelity, although it may take them up to two years to do so. We contrast the Convict results with those from the adjacent Mammoth dialect, in which the local whistle changed substantially over approximately the same period covered by the current study (O'Loghlen et al. 2011) and discuss a possible explanation for the divergent results based on ecological differences between the two dialects. As far as we are aware, this is the first report of such contrasting results for acoustic stability in the songs of contiguous dialect populations. The stability shown by the Convict whistle is remarkable given the extreme structural variation among whistles from other dialect populations in the area and evidence of dispersal and gene flow among these local populations.
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Vol. 115 • No. 3