Validation of the doubly labeled water method in bald eagles (Haliaeetus leucocephalus) and a comparison of two equations for the calculation of energy expenditure.

We validated the doubly labeled water (DLW) technique for measurement of energy expenditure in bald eagles (Haliaeetus leucocephalus) in captivity by simultaneously measuring metabolizable energy intake in a feeding trial. We calculated CO2 production using two equations, one typically used by animal ecologists (the "one-pool" equation) and the other typically used by human nutritionists (the "two-pool" equation). Metabolizable energy intake, as determined by feeding trials, for two adult eagles eating rats averaged 1,160 +/- 89 kJ d-1 and for four nestlings eating fish averaged 2,124 +/- 40 kJ d-1. Energy expenditure measured from DLW turnover using the one-pool equation averaged 2.2% +/- 7.1% higher than metabolizable energy intake measured by feeding trials (not significantly different, P > 0.50), but when the two-pool equation was used, energy expenditure measured with DLW averaged 17.7% +/- 6.7% lower than metabolizable energy intake measured by feeding trials (significantly different, 0.025 < P < 0.05). Thus, the use of the DLW technique with CO2 production calculated by the one-pool equation was validated for bald eagles.