Cutaneous water evaporation--II. Survival of birds under extreme thermal stress.

Two birds, the pigeon (Columba livia) and the partridge (Alectoris chukar), differing in their habits and flight ability were compared with regard to their ability to survive extreme high air temperatures (Ta's). During 270 min survival tests birds were exposed to Ta's between 45 and 60 degrees C and low relative humidities. The pigeon was observed to be unique in its ability to survive 270 min of exposure to 60 degrees C while regulating Tb at 43.8 degrees C. The partridge could not survive 270 min of exposure at Ta's exceeding 48 degrees C. The pigeons were found to be exceptional in their capacity for cutaneous evaporation. Values as high as 20.9 mg H2O/cm-2/hr were measured at 52 degrees C Ta compared to 2.4 mg H2O/cm-2/hr in the partridge. Total evaporation of the pigeon exposed to 56 degrees C Ta was about 20% higher than that in the partridge. Maximum evaporation of the pigeon exposed to 60 degrees C Ta was 34.4 mg H2O/g/hr. The significance of cutaneous water loss for survival during extreme high Ta's is discussed. It is concluded that birds as a class may be divided into two groups with regard to their physiological capacity to withstand heat stress: (1) the majority of studied species employ regular physiological mechanisms and are limited in their cooling capacity to withstand ambient temperatures 48 degrees C; (2) a few avian species, which may be of wide ecological distribution, are equipped with major physiological preadaptations to severe heat stress.