Validation of the doubly labeled water method in growing precocial birds: The importance of assumptions concerning evaporative water loss

The doubly labeled water (DLW) method was validated against respiration gas analysis in growing precocial chicks of the black-tailed godwit (Limosa li mosa) and the northern lapwing (Vanellus vanellus). To calculate the rate o f CO2 production from DLW measurements, Lifson and McClintock's equations ( 6) and (35) were employed, as well as Speakman's equation (7.17) (all singl e-pool models). The average errors obtained with the first two equations (7.2% and -11.6%, respectively) differed significantly from zero but not the error obtained with Speakman's equation (average: -2.9%). The latter error could be reduced by taking a fractional evaporative water loss of 0.13, in stead of the value of 0.25 recommended by Speakman. Ap plication of differe nt two-pool models resulted in relative errors of the DLW method of -15.9% or more. After employing the single-pool model with a fractional evaporativ e water loss value of 0.13, it was found that there was no relationship bet ween the relative growth rate of the chick and the relative error of the DL W method. Recalculation of previously published results on Arctic tern (Ste rna paradisaea) chicks revealed that the fit of the validation experiment c ould be considerably improved by employing a single-pool model and assuming a fractional evaporative water loss of 0.20 instead of the value of 0.50 t aken originally. After employing the value of 0.20, it was found that there was no relationship between the relative growth rate of the chick and the relative error of the DLW method. This suggests that isotope incorporation into new body substances does not cause a detectable error. Thus, the DLW m ethod seems to be applicable in young birds growing as fast as 20% d(-1), a fter making adjustments for the fractional evaporative water loss. We recom mend Speakman's equation (7.17) for general use in growing birds when evapo ration is unknown.