Water influx and efflux in free-flying pigeons

We used tritium-labeled water to measure total body water, water influx (which approximated oxidative water production) and water efflux in free-flying tippler pigeons (Columba livia) during flights that lasted on average 4.2 h. At experimental air temperatures ranging from 18 to 27 °C, mean water efflux by evaporation and excretion [6.3 ± 1.3 (SD) ml · h^-1, n=14] exceeded water influx from oxidative water and inspired air (1.4 ± 0.7 ml·h^-1 n = 14) and the birds dehydrated at 4.9 ± 0.9 ml · h^-1. This was not significantly different from gravimetrically measured mass loss of 6.2 ± 2.1 g · h^-1 (t = 1.902, n = 14, P > 0.05). This flight-induced dehydration resulted in an increase in plasma osmolality of 4.3 ± 3.0 mosmol · kg^-1 · h^-1 during flights of 3-4 h. At 27 °C, the increase in plasma osmolality above pre-flight levels (ΔP(osm) = 7.6 ± 4.29 mosmol · kg^-1 · h^-1, n = 6) was significantly higher than that at 18 °C (ΔP(osm) = 0.83 ± 2.23 mosmol · kg^-1 · h^-1, (t = 3.43, n = 6, P < 0.05). Post-flight haematocrit values were on average 1.1% lower than pre-flight levels, suggesting plasma expansion. Water efflux values during free flight were within 9% of those in the one published field study (Gessaman et al. 1991), and within the range of values for net water loss determined from mass balance during wind tunnel experiments (Biesel and Nachtigall 1987). Our net water loss rates were substantially higher than those estimated by a simulation model (Carmi et al. 1992) suggesting some re-evaluation of the model assumptions is required.