Naturally-occurring deuterium stable isotope ratios can potentially be used to trace water resource use by animals, but estimating the contribution of isotopically distinct water sources requires the accurate prediction of isotopic discrimination factors between water inputs and an animal ( \textr_\textevap /\textr_{\textH2 \textO} ) {\left( {{\text{r}}_{{{\text{evap}}}} /{\text{r}}_{{{\text{H}}_{2} {\text{O}}}} } \right)} supporting the major prediction of the model. The variation we observed in discrimination factors suggests that the apparent fractionation of deuterium will be difficult to predict accurately under natural conditions. Our results show that accurate estimates of the contribution of different water sources to a birds body water pool require large deuterium isotopic differences between the sources.