WATER ECONOMY OF GRANIVOROUS BIRDS - AUSTRALIAN PARROTS

1. In order to understand better the distribution of Australian parrots we tested a model of water regulatory efficiency, based on an ambient temperature (T,) at which metabolic water production (MWP) equals evaporative water loss (EWL), in six species varying in body mass from 27 to 390 g. 2. The relationship of MWP/EWL and T-a has a significant, negative slope for each of the six species. The nature of this relationship is maintained in birds on ad libitum and restricted water and from measurements taken at night. 3. The computed values of T-a at which MWP/EWL=1 also scales inversely, but in a regular manner with body mass. Accepting the premise of the model that higher values of T, at equality reflect higher relative water regulatory efficiency, we contend that smaller parrot species are superior in this regard to larger species. In most cases from the study, dehydration and night-time result in birds achieving an even more favourable state of water balance. 4. To test the predictions of the model within an ecological context we compared the thermal points of water balance for the budgerigar with ambient temperatures in the habitat of the bird in summer and winter. In winter MWP exceeds EWL for 15.5 h each day, but in midsummer MWP is always less than EWL and the birds are dependent on a preformed water source. We suggest that small size in,granivorsus parrots imparts a dependence on small seeds rich in carbohydrates and with maximal yields of metabolic water.