Fig. 5. Heart rate during resting, shallow and deep diving and after exposure to different levels of O₂ and CO₂. Values are means + S.D. from six dives per bird (all dives 18–22 s); the values above of the columns indicate the number of individuals used; resting heart rate was calculated from a 20 min section of ECG recording per bird. For the gas mixtures used, see Materials and methods. *Significantly different from the respective control values (air).