Summary1. Many birds are capable of varying the rate of evaporative heat loss during respiration as a means of temperature regulation.2. Respiratory heat loss becomes increasingly important as air temperatures rise. Above the body temperature of the bird it is the only means of heat loss.3. The rate of respiratory heat loss varies from one species of bird to another. The rate is also dependent on environmental atmospheric humidity.4. The rate of heat loss by respiratory evaporation is dependent on the difference in vapour pressure between that of the respiratory surface and of the inspired air, on the degree of saturation of the expired air, and on the volume of air expired per breath and per unit time.5. There is a constant relation between breathing rate and heat production in the House Sparrow above the upper critical temperature.6. Other things being equal, there is a constant relation between evaporative rate and the vapour pressure difference between the air and the evaporative surface, if one assumes that the vapour pressure of the respiratory surface is equal to that of water at the rectal temperature of the bird.7. When variations due to differences in respiration rate, metabolic rate, atmospheric humidity and body size are excluded, in some species of birds the rate of evaporative heat loss is essentially constant at air temperatures above the upper critical temperature (the upper limit of the thermo‐neutral zone, see p. 114, footnote). In other species the rate of respiratory evaporation increases as air temperatures rise above the upper critical temperature.8. From this evidence and other data in the literature, there appear to be four classes of species: one in which respiratory evaporation as a temperature‐regulating mechanism is unimportant or non‐existent; a second in which respiratory evaporation takes place in the lungs and air sacs and in which, at air temperatures above the upper critical temperature, increased evaporation is achieved only by an increased respiratory rate; a third in which respiratory evaporation takes place in the lungs and air sacs but the evaporative rate can be enhanced without an increase in respiratory rate; and a fourth in which evaporation takes place in the more anterior regions of the respiratory tract such as the trachea or buccal cavity.9. These four groups are interpreted as representing various stages between two extremes of specialization. One extreme is represented by species of cold arid regions in which all or most of the tidal air passes through the lung parenchyma. Respiratory evaporation and hence heat loss is thus minimized. The other extreme is represented by species of hot humid regions in which the major function of ventilation is to provide evaporative heat loss. Respiratory gas exchange is a minor purpose and only a small fraction of the tidal air traverses the lung pas