Observations on the sites of respiratory evaporation in the fowl during thermal panting.

The rate of respiratory water loss (RWL) was investigated in domestic fowls by the open-flow method, and the relative importance of the surfaces of the upper and lower respiratory tract was assessed by cannulating the trachea and by recording the temperatures at the potential evaporating sites. Birds were exposed to Ta from 20 to 40 degrees C and RWL examined at rectal temperatures (Tre) from 41 to 44 degrees C. Overall, the increase in RWL from the whole tract, and from the upper and lower divisions, was by about 1.1, 1.0 and 0.3 mg (g-hr. degrees C)-1, respectively. There was a rapid increase in V and in RWL from the whole and from the upper tract at Tre 41.5-42.5 degrees C, but no comparable change from the lower tract. Temperatures significantly below Ta and Tre (both 43 degrees C) were detected in the trachea and in the nasal and buccal cavities, but not in the air sacs. It was concluded that respiratory evaporation occurs mainly from the upper tract during panting and that the air sacs are unlikely to be involved.