Fig. 2.

Enzyme treatment affects various aspects of song. Whereas the general pattern of air-sac pressure (P; multiple motifs overlaid; horizontal line on bottom trace shows ambient pressure) remains intact, several aspects of the sound (shown as oscillogram; S, spectrogram; F, frequency; and rectified amplitude, A) show effects of enzyme treatment. Examples are shown for 2 birds who received bilateral injections of heparitinase enzyme into the ventral syringeal muscles. The changes include a loss or reduction of high-frequency syllables (denoted by the asterisks), and the sound amplitude of song is visibly reduced 24 h after treatment. rel. units, Relative units.