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

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.