Fig. 5. Air sac pressure (P), tracheal airflow (F) and sound amplitude (A) during song were increasingly reduced after one (red) and two (green) injections into the posterior thoracic air sacs, as compared with pre-injection (blue). A representative song is also shown spectrographically before and after two injections. During the first and second expiratory pulse, the bird closed its syrinx (as indicated by zero tracheal flow; arrows), resulting in elimination of these sound segments. Quiet respiration changed after injections into the posterior thoracic air sacs. The orange horizontal line depicts ambient pressure.