Energetics of Growth in Nestling Savannah Sparrows: A Comparison of Doubly Labeled Water and Laboratory Estimates

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Abstract
The energy expenditure of free-living nestlings has previously been estimated by any of several methods, each of which contains assumptions that may result in errors of uncertain magnitude. We evaluated the energy budget of nestling Savannah Sparrows (Passerculus sandwichensis) by analyzing their oxygen consumption and their accumulation of energy in tissues during growth. These results were compared with those obtained by means of the doubly labeled water (DLW) technique. From data on oxygen consumption of individual nestlings in metabolism chambers at 35.degree. C, we calculated that a neonate metabolized 110.4 kJ of energy during the eight days that it occupied a nest. Applying standard conversion factors, we determined that a nestling accumulated 20.0 and 61.6 kJ of energy in lipid and non-lipid dry matter, respectively, for a total production (P) of 81.6 kJ. Direct evaluation of the energy in tissues by means of bomb calorimetry indicated that nestlings actually accumulated 75.2 kJ of energy, a value 7.8% lower than we previously obtained by converting lipid and non-lipid dry matter to energy with accepted conversion factors. From our analysis using data obtained in the laboratory, total metabolized energy (TME) equaled 185.8 kJ. Field metabolism, as estimated by DLW, summed to 171.5 kJ for the 8-day nestling period whereas estimates from oxygen consumption in the laboratory amounted to 110.4 kJ, a 35.6% difference. This disparity may have resulted from an increaased energy expenditure for thermoregulation and activity during the latter stages of the nestling period, components which are not measured in laboratory experiments. Total metabolized energy (TME) as estimated by DLW (246.7 kJ) differed from laboratory estimates (182.9 kJ) by about 25%. Production (P) to TME ratios for Savannah Sparrows on a daily basis were nearly as high as those for altricial embryos (Agapornis) with values declining from a maxima of 0.4-0.5 to 0.1 near the end of the nestling period.