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 pre dictions show that evaporative water loss (EWL) depends on air temperature (T-a) and water vapor density (rho(a)), but these relationships have not be en 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 th is assumption is untested. To study the dependence of water losses on envir onmental conditions in free-flying birds and to quantify the contribution o f excretory water loss to total water loss, we estimated evaporative and ex cretory water losses in 16 trained, free-flying tippler pigeons (Columba li via, 250-340 g). We collected excreta by attacking a light latex, water-imp ermeable receptacle around each bird's vent. By gravimetry, we measured eva porative 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 flig hts. EWL was constant with respect to T-a when less than 15 degrees C, and increased with increasing T-a above 19 degrees C, indicating that evaporati ve cooling was invoked when the heat load increased. EWL increased with inc reasing rho(a), possibly due to the strong correlation between rho(a) and T -a. Excretory water loss was independent of rho(a) or T-a and averaged almo st 10% of the total water loss. Measurements of EWL made on pigeons during wind tunnel experiments and previous free-flight studies are consistent wit h our free-flight measurements made at similar T(a)s.