Song sparrow trill performance varied between study regions.

<p><b>A</b>) Trill rate-frequency bandwidth trade-off of song sparrows. The regression line is calculated from all recorded trills, and each point represents a randomly chosen trill type of one individual song sparrow. The dotted lines represent the trill rate-bandwidth cut-off of 4 kHz distance from the trill performance regression line (solid). Low performance trills are between the origin and the lower dotted line. <b>B</b>) The proportion of song sparrows singing high performance (grey, distance from regression line<4 kHz) or low performance (black, distance from regression line>4 kHz) trills by region. The numbers in bars are the actual percentages of individual sparrows singing low performance trills.