Fig. 2. Air sac, arterial and venous P_O2 profiles during shallow (less than 30 m), 3-min dives in three different birds. P_O2 profiles from the air sac (EP 05) (Stockard et al., 2005) and aorta (EP 02) demonstrate initial compression hyperoxia and then a gradual decline in P_O2 secondary to air sac O₂ depletion and the decrease in ambient pressure during ascent. Vena caval P_O2 slowly increased during the dive of EP 20. The P_O2 data were recorded at 15 s intervals. Dives began at 0 time. Prior to the onset of the dive for the arterial profile, the bird had just surfaced for a breath after a prior dive.