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 bird s as a function of the bird's morphological and physiological characte ristics and of ambient meteorological conditions. Sensitivity analysis indicated that, of the model input variables, oxygen extraction and e xhaled air temperature have the largest effect on dehydration during f light. Oxygen extraction affects dehydration rate through its effect o n ventilatory volume and, thus, on respiratory water loss. Exhaled air temperature influences dehydration rate by modulating the recondensat ion of water vapor in exhaled air on the walls of the respiratory pass ages. This might save water that would otherwise be lost. Both variabl es depend on other factors, such as the structure of the air passages, the flow dynamics of respiratory air, the neural control of blood flo w to the mucosal surfaces of the anterior respiratory tract, and the b lood oxygen-carrying capacity.