Temperature Regulation in Turkey Vultures

Temperature regulation by six Turkey Vultures (Cathartes aura) was studied at ambient temperatures (Ta) from 11 to 40°C in a metabolic chamber. Within this range body temperature varied little, averaging 39.7°C, and the ratio of CO2 production to O2 consumption averaged 0.76. The thermal-neutral zone (TNZ) extended from 26 to 40°C. Below 26°C evaporative water loss and the heat-transfer coefficient were minimal, reflecting maximum insulation, and breathing and heart rates remained nearly constant at 10 and 142 min-1, respectively. Oxygen consumption increased by 12 μl O2 STPD/(g hr) per °C of decrease in Ta below 26°C, so the percentage of metabolic heat lost by evaporation decreased from 22 to 13%. Within the TNZ O2 consumption was minimal, averaging 0.73 ml STPD/(g hr), but evaporation increased exponentially, dissipating metabolic heat to a maximum of about 90% at 40°C. Insulation decreased exponentially within the TNZ. Despite this, however, the birds underwent rapid hyperthermia above 40°C. The likely explanation for this is that under the present experimental conditions the Turkey Vultures could not make use of important behavioral components of their thermoregulatory repertoire, including neck extension, wing spreading, and urination on the legs. Were this not so they likely would have exhibited greater reduction in insulation and tolerated higher Ta. The thermal relations of Turkey Vultures were similar to those calculated from allometric relations and to those measured previously in the closely related, partially sympatric Black Vulture (Coragyps atratus).