Figure 1

A short section of skylark song (waveform in top panel and spectrogram in bottom panel) showing some acoustic traits that are difficult to account for in metrics of song performance: for example, fast amplitude modulations (syllable at second 1.5), prominent harmonics (at second 2), multiple frequency inflections within the syllables (at second 3.3), or differences in amplitude across syllables (top panel). A new metric of performance avoids these difficulties by instead assessing the speed of frequency changes during the silent gaps between syllables (red lines).