METABOLISM AND WATER FLUX OF CAPTIVE AND FREE-LIVING AUSTRALIAN PARROTS

Occupation of desert environments often requires evolutionary specialisations that minimise food and water requirements. One physiological adjustment to living in a hot, dry climate that has been found in several laboratory studies of birds is a reduced basal metabolic rate (BMR), which often translates into a diminished rate of evaporative water loss (EWL). In free-living birds, these physiological traits are thought to result in a lower field metabolic rate and water flux. We studied metabolism and water flux of a number of species of Australian parrots, both in the laboratory and in the field. After combining our laboratory data with values from the literature, we performed allometric analyses to search for evolutionary specialisation in metabolism and water flux in desert-adapted parrots. Our data do not support the idea that parrots living in arid environments have a reduced BMR. Field metabolic rates of parrots from western Australia were indistinguishable from those of other nonpasserine birds. Laboratory EWL was significantly lower for parrots living in desert environments than for those occupying more mesic habitats, and often lower than that expected from body size. Some species of parrots that live in desert regions of Australia have evolved mechanisms that reduce EWL, but this does not involve a reduction in BMR. In the field, parrots living in Western Australia had a lower water influx than predicted for nonpasserines, but this did not approach the value often found in other desert-adapted species. Values for the water economy index (water flux in free-living animals relative to their energy metabolism) were among the lowest that have been reported for desert-adapted birds.