The ins and outs of water balance in free-flying pigeons

When birds take flight, flapping their wings, they increase their metabolic rates 10 to 15 times over their resting metabolic rates. Small passerines may cross wide ecological barriers such as the Mediterranean Sea, or even the Sahara desert, in a nonstop flight that can last 8 hours or more. Such prodigious feats motivated scientists to ask themselves what are the physiological limitations to long-duration flapping flight. Of the limitations that come to mind - for example, energy, water, or protein stores, or even the need to sleep - my associates and I have concerned ourselves primarily with water. We used tippler pigeons - a breed of homing pigeon with a propensity to fly for hours at a time in large circles above its loft - as a model to study the avenues of water flux in free-flying birds. Water gain and loss were assessed in birds flying over a wide range of ambient temperatures using isotope-labeled water. We used gravimetry to separate excretory and evaporative water losses, and separated total evaporative water loss into respiratory and cutaneous components using radio telemetry to measure respiratory loss. Inflight body temperature was also measured by telemetry. Finally, we assessed kidney function that is one of the primary mechanisms involved in saving water during flight. We conclude that as our knowledge about physiological function in birds increases, the more it would appear that the selective pressures associated with the physiological requirements for flight have "pre-adapted" birds for living and functioning in other stressful environments. Excess heat must be lost without unnecessary evaporative water loss and metabolic output must be optimized. These are the same attributes necessary for survival in deserts and other desiccating conditions.