Evaporative and excretory water loss during free flight in pigeons

Body water conservation is important in flying birds because the very high metabolic demands and heat dissipation requirements during flight depend on plasma-volume integrity. Wind tunnel experiments and theoretical model predictions show that evaporative water loss (EWL) depends on air temperature (T(a)) and water vapor density (ρ(a)), but these relationships have not been examined in free-flying birds. The contribution of excretory water loss to the total water loss of a flying bird is thought to be negligible but this assumption is untested. To study the dependence of water losses on environmental conditions in free-flying birds and to quantify the contribution of excretory water loss to total water loss, we estimated evaporative and excretory water losses in 16 trained, free-flying tippler pigeons (Columba livia, 250-340 g). We collected excreta by attaching a light latex, water-impermeable receptacle around each bird's vent. By gravimetry, we measured evaporative and excretory water losses of birds for eight flights at different T(a)s and compared flying to resting (control) birds for two of these flights. EWL was constant with respect to T(a) when less than 15 °C, and increased with increasing T(a) above 19 °C, indicating that evaporative cooling was invoked when the heat load increased. EWL increased with increasing ρ(a), possibly due to the strong correlation between ρ(a) and T(a). Excretory water loss was independent of ρ(a) or T(a) and averaged almost 10% of the total water loss. Measurements of EWL made on pigeons during wind tunnel experiments and previous free-flight studies are consistent with our free-flight measurements made at similar T(a)s.