1. The field metabolic rate (FMR) of an endothermic animal represents its energy expenditure in a natural environment, or its energy budget, and its field water flux (FWF) reflects the animal’s water requirements.

2. We examined FMR of 103 species and FWF of 75 species of adult birds from direct field measurements by the doubly-labelled water method, and used the phylogenetic generalised least squares method to conduct a phylogenetically-informed, comprehensive analysis of the relationship between FMR, FWF, and multiple environmental and biological variables.

3. FMR was strongly positively associated with body mass with an allometric exponent of 0.66, and seabirds had lower FMR than terrestrial species. Birds consuming plant matter had lower FMR compared to omnivores, carnivores or nectarivores, and low ambient temperature was associated with higher FMR. There was little evidence for phylogenetic covariance in FMR, even though previous studies identified a phylogenetic signal for BMR. Life history traits, such as fecundity and migration, were also not strongly associated with FMR.

4. FWF was strongly positively associated with body mass with an allometric exponent of 0.61, and was strongly related to precipitation but not to temperature. Diet and habitat use had significant effects on FWF, with nectarivores and marine species exhibiting higher values than granivores and forest birds.

5. Thus, FMR and FWF are affected similarly by body size and differently by environmental temperature and precipitation, while the roles of diet, life history traits and habitat are more nuanced and generalities remain elusive.