Validation of the Doubly Labeled Water Technique for Measuring Energy Metabolism in Savannah Sparrows

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Abstract
We made simultaneous measurements of CO₂ production by Savannah sparrows (Passerculus sandwichensis) using doubly labeled water (DLW) and gravimetric (GM) methods in order to assess the accuracy of DLW measurements in these birds. The average DLW value was 6.5% higher than the GM value (4.26 ± 0.54 [SD] vs. 4.00 ± 0.47 g CO₂/day, n = 7). This difference is small but statistically significant (P < .01, paired t-test). A small systematic error in the GM method may account for this discrepancy. Labeled water injected into a pectoral muscle equilibrated with body water within 1 h. Body water volumes, calculated as dilution space in injected birds (61.7% of live body mass) agreed well with measurements made by drying four birds that were not injected (61.1%). Thus, estimates of metabolic rate and body water volume made with isotopically labeled water are reasonably accurate in these birds.