FIELD METABOLISM AND WATER REQUIREMENTS OF SPINIFEX PIGEONS (GEOPHAPS-PLUMIFERA) IN WESTERN-AUSTRALIA

Spinifex pigeons (Geophaps plumifera) are one of the few avian species that have evolved the capacity to reside in the hot and dry regions of central and north-western Australia. Previous investigation has revealed that their basal metabolic rate (BMR) equals only 68% of allometric prediction. In this study, we addressed the hypothesis that these birds have a reduced field metabolic rate (FMR) and water influx as a result of their lowered BMR. We measured the FMR and water flux of free-living spinifex pigeons by means of the doubly labelled water method. Although body mass of free-living male and female pigeons differed significantly, with males weighing on average 90.8 +/- 7.7 g (+/- s.d.) and females 80.2 +/- 5.6 g, FMR was statistically indistinguishable between sexes. For sexes combined, FMR averaged 139.9 mL CO2 h-1, or 73.5 kJ day-1, a value 38.7% of allometric expectation. These data support the hypothesis that spinifex pigeons have a markedly reduced FMR, probably, in part, the result of a depressed BMR compared with other birds of similar size. Our phylogenetic analysis of the BMR of pigeons lacked sufficient data to determine whether a reduced BMR in Australian pigeons was the consequence of ecological adaptation or phylogenetic constraint. Water influx ranged from 2.5 to 39.0 mL day-1 and averaged 18.4 mL day-1. Of the total water intake, 83.5% came from drinking; their food, seeds, supplied about 4%. Maintenance metabolism, energy allocated to basal plus thermoregulatory metabolism, accounted for about 67% of the average FMR, indicating that the activity requires relatively low energy expenditure in these birds.