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Song repertoire of the bobolink: A reassessment

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Previous studies have reported that male bobolinks, Dolichonyx oryzivorus, sing only two stereotyped song types. Here we reassess this description of the bobolink repertoire, using methods of sampling and classifying songs better suited to describing song diversity. Recording the songs of 51 males from five prairies in Iowa, we examined within- and between-individual variation for each of three levels of bobolink singing organization: phrases, phrase sequences and repertoires. A single song was composed of 1-15 phrases selected from about 10 locally used phrase types. Shared phrases were used with similar frequencies among different males at a locality. Two-, three- and four-phrase sequences were used repeatedly in the song variants produced by individual males. Unlike other investigators, we considered each song unit composed of a unique sequence of phrases to be a different song variant. Some individuals had two basic song patterns which were varied to create an assortment of song variants. Other individuals had repertoires too complex to be described by two basic patterns. Eighteen males for which we obtained samples of 20 or more songs sang between 8 and 43 different song variants. Computing an index of singing versatility derived from information theory, we found that these individuals varied considerably in the versatility with which they delivered their song variants. Males with high versatility used a greater variety of phrase transitions and repeated song variants less often than males with low versatility. This method of describing song repertoires and singing versatility provides the potential to test hypotheses regarding the role of the bobolink song repertoire in communication and mate choice.

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