Validation of the doubly labeled water technique for measuring energy metabolism in starlings and sparrows

I have tested the idea that doubly labeled water (DLW) can accurately predict CO2 production in savannah sparrows, song sparrows, white-throated sparrows, starlings, and a single house sparrow by comparing DLW estimates with those obtained simultaneously by capturing expired CO2 in Ascarite. In addition I used the energy balance method to see if metabolic rates generated from DLW measurements accurately reflected the actual metabolic rates of these birds. I found close agreement in DLW and the gravimetric and energy balance methods, with DLW underestimating CO2 production on average by -3.5% in sparrows, and -7.1% in starlings. Similarly, the energy balance method indicated a -3.1% underestimate by DLW for sparrows and a -5.1% for starlings. The DLW method can yield reasonable estimates of CO2 production in a variety of passerine birds.