Environmental and ecological correlates of avian field metabolic rate and water flux

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. We examined FMR of 103 species and FWF of 75 species of adult birds from direct field measurements using the doubly labelled water method, and used the phylogenetic generalized least squares method to conduct a phylogenetically informed, comprehensive analysis of the relationship between FMR, FWF and multiple environmental and biological variables. Field metabolic rate 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 basal metabolic rate. Life-history traits, such as fecundity and migration, were also not strongly associated with FMR. Field water flux 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. 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. A free Plain Language Summary can be found within the Supporting Information of this article.