Why do meadowlarks switch song types?

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Abstract
Singing of eastern and western meadowlarks, Sturnella magna and S. neglecta, was monitored during 85-min experiments in which 13 1-min bouts of recorded conspecific song were broadcast at irregular intervals. Both species tended to switch song types at the onset of playback. Song bouts of magna were shortened in playback periods, with multiple switches occurring; bouts of neglecta during playback were of normal length. Overall, bouts of magna were shorter than those of neglecta. When playback began, long bouts were more likely to end in a switch than shorter ones. Survivorship curves of completed bouts for both species followed a log-linear function; beyond a minimum length switching was independent of previous bout length. Lengths of successive bouts were positively associated in birds with short bouts (mainly magna) but not in birds with longer bouts (mainly neglecta). Switching was associated with long interbout intervals and with movements, although most switches occurred in short intervals without movements. These results are mainly consistent with the hypothesis that delivery of repertoires reduces habituation of listeners. This antihabituation mechanism is distinguished from other explanations for repertoires. A derivative explanation, that song variety serves as a graded signal of readiness to interact (inter- or intra-sexually), is suggested and predictions are discussed.