Fig. 7. Zebra finch W42 showed a consistently increasing difference in air sac pressure (shown in direct comparison, P, and below as percent decrease from pre-injection values; colors as in Fig. 5) amplitude throughout each expiratory pulse. Song output during all three treatments is shown as amplitude traces (A; rectified and integrated) and as spectrograms (pre-injection and two injections) and illustrates how particularly high-frequency syllables are reduced in amplitude after the injections. Each of the two sets of high-frequency syllables is a combination of one element produced at the end of an expiratory pulse and the second during the following inspiration (phonatory minibreath). In both sets, the expiratory element disappeared (almost zero amplitude) whereas the inspiratory part was strongly reduced in amplitude.