Development of Thermoregulation and the Energetics of Nestling Oilbirds (Steatornis caripensis)

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Abstract
We measured body temperature ( )and mass-specific O₂ consumption ( )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 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 at 39.3° ± 0.6° C. Small chicks (50-150 g) had between 2.378 ± 1.167 and 2.570 ± 1.154 mL O₂ · (g · h)⁻¹ at ambient temperature ( )< 30° 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․ °C)⁻¹, and declined from 1.673 ± 0.185 to 1.337 ± 0.178 mL O₂ · (g · h)⁻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 at 20° C. Using data on , 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.

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