Tufted ducks were trained to dive to and from a respirometer box on a 1.7 m dive tank, so that measurements of respiratory gas exchange could be measured, along with time budget data. These data were combined with power cost estimates of diving to show that the optimal breathing model quantitatively predicted surface duration and the oxygen metabolised during foraging for the mean of all subject ducks but not for individual birds. Respirometry data also showed that both the oxygen and carbon dioxide stores were close to full adjustment after mean surface duration suggesting they have a similar influence on surface duration in tufted ducks, while pre-dive hyperventilation caused hypocapnia suggesting carbon dioxide is more often a limiting factor on dive duration. Oxygen uptake was not affected by hypercapnic exposure between dives and minimally affected by hypoxia, however dive time budgeting changed in both cases. This confirmed an influence of carbon dioxide on diving behaviour while estimates of respiratory exchange ratios above one during dives from hypoxia suggested the employment of anaerobic metabolic pathways in hypoxic conditions. Allometric studies investigating relationships between body mass and diving parameters across and within taxonomic groups of divers highlighted a number of limitations in our current knowledge of diving animals and also questioned some of the mass associated correlations that have been widely considered to exist across diving species.