Urinary water loss during free-flight in pigeons

We examined urinary water loss in free-flying birds using trained tippler pigeons. In each pigeon we implanted an osmotic pump that continuously secreted 3H-polyethyleneglyco! (a glomerular filtration marker in birds). In two 4 5 h flights (10 birds in winter at an ambient temperature of 13-15°C and 7 in summer at 23°C), we made measurements of preflight (hydrated, resting control birds) and post-flight 3H-PEG activity and osmolality in blood and ureteral urine. For comparison, we made the same measurements in the 10 winter birds before and after dehydration under controlled conditions (12 h at 30°C). Urine osmolality increased significantly up to 3 times the control level in both post-flight and dehydrated pigeons, urine plasma osmolality ratios did not exceed 2. Compared to controls, GFR was significantly lower after flight in summer, but did not change in either post-flight or dehydrated winter pigeons. In winter, mean post-flight urine flow rate (UFR) decreased significantly to less than half the control level, while in summer, post-flight UFR was not different from control. In general, mean filtered water reabsorption (FrH2O) increased from 95% in controls to 98 % in post-flight and dehydrated birds. The data from this first study of kidney function during flight are consistent with previous studies of dehydration in birds and exercise in mammals in which both increased FrHjO and decreased GFR contribute to renal conservation of water.