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Vocal Dialects and Their Possible Relation to Honest Status Signalling in the Brown-Headed Cowbird

Stephen I. Rothstein and Robert C. Fleischer
The Condor
Vol. 89, No. 1 (Feb., 1987), pp. 1-23
DOI: 10.2307/1368756
Stable URL: http://www.jstor.org/stable/1368756
Page Count: 23
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Vocal Dialects and Their Possible Relation to Honest Status Signalling in the Brown-Headed Cowbird
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Abstract

We describe dialects in the flight whistle of the Brown-headed Cowbird and contrast previous hypotheses for the maintenance of dialects with a new one that assumes that a male's ability to give the correct local dialect is an honest signal of relatively high male quality. The three dialects upon which we focus are part of an extensive dialect system along the eastern Sierra Nevada. The dialects are partially isolated by unsuitable habitat and are unusual because they differ via lexical rather than the less extreme phonetic differences that characterize song dialects in most other species. Because males whistle just before copulating, whistles may function in mate choice. Since flight whistles are also used over long distances, we predicted and confirmed that males within the same dialect have quantitative whistle differences of potential value for individual recognition. Other quantitative analyses indicated phonetic differences among homologous whistle elements from adjacent dialects. Recordings made between 1978 and 1980 showed no quantitative or qualitative differences from a large sample of 142 males recorded between 1983 and 1985. Such temporal stability is expected since the dialects are large, being 10 to 30 km in extent and probably contain hundreds of individuals. Historical records demonstrate that the dialect populations developed since the late 1930s. Males banded in one dialect but recorded in another made up 13.0% of our adult sample and were more likely to have foreign whistles than adults banded and recorded in the same dialect. Yearling males were significantly less likely to whistle than adults, and yearlings that did whistle were significantly more likely to have foreign whistles. Thus possession of a locally appropriate whistle is a reliable indicator of a male's age which is a major correlate of male mating success and possibly of male quality, as nearly all copulations involve adult males. These age differences are consistent with our new "honest convergence" hypothesis and inconsistent with the local or genetic adaptation hypothesis which predicts that vocal ontogeny is closed by a male's first breeding season. Bilingual males and those with hybrid whistles combining elements from two dialects made up 0 to 8% of the males within dialect areas. By contrast, 38% of males in one contact zone between two dialects were bilingual and 54% of males in another contact zone gave hybrid whistles. These trends are consistent with the honest convergence hypothesis and inconsistent with hypotheses that dialects are maintained by isolation or because they contribute to local adaptation among populations.

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