The early ontogeny of ventilation and homeothermy in an altricial bird, Agapornis roseicollis (Psittaciformes).

The body temperature (Tb) of brooded Agapornis nestlings increased with age from 36.2 on the day of hatching to 38.9 degrees C on day 11.5, whereas the thermally neutral ambient temperature (Ta,n) decreased with age. The Tb of nestlings after a 20 min exposure to Ta approximately equal to 5 degrees C less than Ta,n increased from 31.9 on day 0.5 to 37 degrees C on day 11.5. At Ta,n both resting VO2 and mass-independent resting VO2 increased with age. Ventilation frequency (f) at Ta,n increased with age from 14.9 br/min on day 0.5 to 65.6 on day 11.5. This trend is the opposite of the allometric prediction based upon data from adult birds. At hatching, tidal volume (VT) was larger than predicted allometrically, whereas minute volume (VI) was smaller. After 20 min exposure to Ta 5 degrees C less than Ta,n, f, VT, and VI in nestlings 1 to 3 days old remained unchanged, but VO2 and oxygen extraction efficiency decreased. This mild cold-challenge caused a transient increase in VO2 of nestlings 5 to 8 days of age and 20 to 35% of adult mass. By the time the nestlings were 11.5 days old and had a mass 44% that of adults, their metabolic response to a mild cold-challenge was equal to that of an adult of the same mass. However, they were not capable of effective homeothermy over a wide range of Ta's.