Water as a physiological limitation to flight duration in migrating birds: The importance of exhaled air temperature and oxygen extraction

We used a computer model that calculates net water loss of flying birds as a function of the bird' s morphological and physiological characteristics and of ambient meteorological conditions. Sensitivity analysis indicated that, of the model input variables, oxygen extraction and exhaled air temperature have the largest effect on dehydration during flight. Oxygen extraction affects dehydration rate through its effect on ventilatory volume and, thus, on respiratory water loss. Exhaled air temperature influences dehydration rate by modulating the recondensation of water vapor in exhaled air on the walls of the respiratory passages. This might save water that would otherwise be lost. Both variables depend on other factors, such as the structure of the air passages, the flow dynamics of respiratory air, the neural control of blood flow to the mucosal surfaces of the anterior respiratory tract, and the blood, oxygen-carrying capacity.