WATER ECONOMY OF GRANIVOROUS BIRDS - AUSTRALIAN PARROTS

1. In order to understand better the distribution of Australian parrot s we tested a model of water regulatory efficiency, based on an ambien t temperature (T,) at which metabolic water production (MWP) equals ev aporative water loss (EWL), in six species varying in body mass from 2 7 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 relatio nship is maintained in birds on ad libitum and restricted water and fr om 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 ma ss. Accepting the premise of the model that higher values of T, at equ ality reflect higher relative water regulatory efficiency, we contend that smaller parrot species are superior in this regard to larger spec ies. In most cases from the study, dehydration and night-time result i n birds achieving an even more favourable state of water balance. 4. T o test the predictions of the model within an ecological context we co mpared the thermal points of water balance for the budgerigar with amb ient 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 al ways less than EWL and the birds are dependent on a preformed water so urce. We suggest that small size in,granivorsus parrots imparts a depe ndence on small seeds rich in carbohydrates and with maximal yields of metabolic water.