Soorim Song, Steven R. Beissinger
Throughout its life, a wild bird must balance energy intake, maintain its body temperature within a range that requires little additional energy beyond basal metabolic rate, and conserve water according to the environmental conditions it encounters. The energy and water used by an animal in the field are its field metabolic rate (FMR) and field water flux (FWF), respectively. Understanding how FMR and FWF are associated with species’ traits and the environments they inhabit has important implications for how animals balance their energy and water budgets under climate change. Previous studies have explored the relationship between FMR and subsets of factors that affect it, but there has not been a comprehensive examination of the factors affecting the variation in FWF among bird species.
We examined FMR of 103 species and FWF of 75 species of adult birds from direct field measurements made by the doubly-labelled water method, which uses isotopic tracers to quantify the rate of energy expended. We conducted a comprehensive analysis of the relationship between FMR, FWF, and multiple environmental and biological variables using an approach that also controlled for the evolutionary relatedness among bird species. Based on past studies, we expected FMR in birds to be: (1) positively related to basal metabolic rate (BMR), body size and clutch size; and (2) higher for species that engage in costly flying behaviours to find food or are provisioning young, and that live in environments with lower ambient temperature and higher water availability. As birds lose water mainly through waste elimination and evaporation, we expected higher FWF would be associated with locations with higher ambient temperatures and water availability, and with higher dietary water content. We also expected that marine birds would have lower FWF than terrestrial species.
FMR was positively associated with increasing body mass and was higher for species inhabiting cooler locations. Seabirds had lower FMR than terrestrial species, and birds consuming plant matter had lower FMR compared to omnivores, carnivores or nectarivores. But, there was little evidence that evolutionary relatedness affected FMR (i.e., closely-related species were not more likely to have similar FMRs than distantly-related species).
FWF increased strongly with body mass, but was related to precipitation and not to temperature. Diet and habitat had significant effects on FWF, with nectarivores and marine species exhibiting higher values than granivores and forest birds.
Other species’ traits had little influence on FMR or FWF. Interspecific differences in FMR and FWF were unrelated to breeding phase, clutch size, or migratory behaviour.
In summary, general patterns for FMR and FWF support the importance of body size and climatic variation, while the roles of diet, life history traits and habitat remain more elusive.