Respiratory water loss in free-flying pigeons

We assessed respiratory and cutaneous water loss in trained tippler pigeons (Columba livia) both at rest and in free flight. In resting pigeons, exhaled air temperature T(ex) increased with ambient air temperature T(a) (T(ex)=16.3+0.705T(a)) between 15 degrees C and 30 degrees C, while tidal volume V(T) (V(T)=4.7+/-1.0 ml, mean +/- S.D. at standard temperature and pressure dry) and breathing frequency f(R) (f(R)=0.46+/-0.06 breaths s(-1)) were independent of T(a). Respiratory water loss, RWL, was constant over the range of T(a) (RWL=1.2+/-0.4 mg g(-1) h(-1)) used. In flying pigeons, T(ex) increased with T(a) (T(ex)=25.8+0.34T(a)), while f(R) was independent of T(a) (f(R)=5.6+/-1.4 breaths s(-1)) between 8.8 degrees C and 27 degrees C. Breathing frequency varied intermittently between 2 and 8 breaths s(-1) during flight and was not always synchronized with wing-beat frequency. RWL was independent of air temperature (RWL=9.2+/-2.9 mg g(-1) h(-1)), but decreased with increasing inspired air water vapor density (rho(in)) (RWL=12.5-0.362rho(in)), whereas cutaneous water loss, CWL, increased with air temperature (CWL=10.122+0.898T(a)), but was independent of rho(in). RWL was 25.7-32.2 %, while CWL was 67.8-74.3 % of the total evaporative water loss. The data indicate that pigeons have more efficient countercurrent heat exchange in their anterior respiratory passages when at rest than in flight, allowing them to recover more water at rest at lower air temperatures. When evaporative water loss increases in flight, especially at high T(a), the major component is cutaneous rather than respiratory, possibly brought about by reducing the skin water vapor diffusion resistance. Because of the tight restrictions imposed by gas exchange in flight, the amount of water potentially lost through respiration is limited.