Water loss during free-flight in pigeons

Water conservation is especially important for a flying bird because demands for sufficient oxygen supply, heat dissipation and metabolite exchange increase greatly during flight, requiring maintenance of plasmavolume integrity. Reducing evaporative water loss (EWL) means losing less heat, but decreasing excretory water loss (EXWL) would save water without influencing heat balance. Thus, we hypothesized that during flight, EWL will increase with increasing air temperature, but birds will maintain low EXWL. We measured EWL and EXWL in 20 trained, free-flying tippler pigeons (Columba livia, 250-340g). We collected excreta by attaching a light rubber, water-impermeable bag around each bird's vent. Birds and bag were weighed before and immediately after flights of 3 5-5 h; bag contents were dried and reweighed after flight. Measurements were made for 6 flights at air temperatures (T.) of 13-24°C; at T,=14°C and T,=19°C, the same measurements were made on 6 resting birds. In flyers, EWL increased from a mean of I4.2±2.6 mgfHjOg1 (body mass)-h' (N=12) at 13°C to 35.512 9 mg-g'-h1 at 24°C (N=5), while EXWL varied widely, but was not correlated with T. (range 0.5-4.0 mg-g'h'). Therefore, the significant increase in total water loss with increasing T, was largely due to the increase in EWL. EWL was significantly greater in flying than in resting birds; EXWL was not and was independent of T.. The results corroborate our hypothesis. (Supported by United States-Israel Binational Science Foundation grant # 93-00232).