DEVELOPMENT OF THERMOREGULATION AND THE ENERGETICS OF NESTLING OILBIRDS (STEATORNIS-CARIPENSIS)

We measured body temperature (T(b)) and mass-specific O2 consumption (VO2) Of nestling oilbirds (Steatornis caripensis) ranging in mass from 14 to 600 g and in age from 1 to 83 d at Cueva del Guacharo, Venezuela. Chicks became progressively more able to regulate T(b) at body masses between 95 and 250 g and attained 75% of the adult level of thermal competence at about 150 g or an age of 21 to 22 d. At body mass > 300 g nestlings were thermally competent and maintained T(b) at 39.3-degrees +/- 0.6-degrees-C. Small chicks (50-150 g) had VO2 between 2.378 +/- 1.167 and 2.570 +/- 1.154 mL O2.(g.h)-1 at ambient temperature (T(a)) > 30-degrees-C At masses of 250-550 g prior to weight recession, mean wet thermal conductance (C) declined from 0.145 to 0.101 mL O2.(g.h.degrees-C)-1, and VO2 declined from 1.673 +/- 0.185 to 1.337 +/- 0.178 mL O2.(g.b)-1 within the thermoneutral zone. Metabolic measurements indicated that groups of two and three nestlings benefited from a 9.1% and a 19.7% reduction, respectively, in VO2 at 20-degrees-C Using data on VO2, the effect of siblings, and parental attendance, we built a profile of the energy expenditure of chicks through the nestling period. Energy expenditure increases rapidly to 210.8 kJ.d-1 at 250 g but increases only slowly thereafter to peak at 372.6 kJ.d-1 at 500 g. The rate of mass loss during the weight recession period corresponds closely with that predicted by the metabolism of fat reserves. We use our measures of energy expenditures in conjunction with published data on diet and fat reserves to evaluate energy balance and the reasons for fat deposition.