Fig. 3. One bird displayed distinct periods of apnea after song bouts. These periods shortened after the first injection and were shortened or no longer present after the second injection. (A) Air sac pressure traces of song bout before injection (blue) and after two injections (green). (B) Apnea duration before injections (blue circles) was positively correlated with bout duration (linear regression, r=0.65, P=0.0059), but injections reduced or eliminated periods of apnea (red, one injection; green, two injections).