Nestlings less than six days old were ectothermic at ambient temperatures between 15 and 35 degrees C. Shivering was absent and panting sporadic and awkward. 2. At 7-14 days of age, shivering and panting responses were evident but body temperature was labile. 3. At 20 and 30 degrees C the rate of oxygen consumption increased with age to a peak of 3.5 ml/g per hr then decreased to around 2 ml/g per hr in the 400 g nestlings. At 40 degrees C the larger nestlings had a higher Vo2 than the younger birds. 4. Evaporative water loss (EWL) at Ta of 20-30 degrees C increased from approx 1 mg/g per hr in two-day-old ravens to 3-6 mg/g per hr in 11-20-day-old birds. At 40 and 45 degree C, EWL increased from approx 5 mg/g per hr in young nestlings to 10-34 mg/per hr in birds more than 11 days old. 5. Differentials in Vo2 between 20 and 30 degrees C show that the transition from ectothermy to endothermy occurred rapidly and completely in 12-14-day-old ravens. 6. At the onset of physiological thermoregulation the Vo2 of nestling birds was nearly twice that of a hypothetical adult. 7. The dry heat transfer coefficient decreased as weight increased indicating that young birds dissipate more heat passively than do older birds. Older birds dissipate heat predominantly by evaporative cooling.
