Fig. 4.

(A) Example of arrival of helium in the trachea and the posterior thoracic (PTAS) and interclavicular air sacs (ICAS) in a quietly breathing yellow-headed blackbird. After passing the tracheal flow probe, helium arrives with a delay first in the PTAS (arrow) and then in the ICAS (arrow). Abbreviations and labels as in Fig. 2. (B) The more helium (quantified as integrated voltage of the tracheal flow signal during the passage of helium) enters the respiratory system, the longer its presence in the PTAS and ICAS. Data show one example for a yellow-headed blackbird.