In this chapter I shall discuss the energy expenditure for animal activity, the effects of activity on the heat balance, and the need to use evaporation of water to keep cool.

Air-cooled flying machines

To conserve weight, man-made aircraft use air-cooled engines. No engineer would dream of loading an airplane with water to cool the engines by evaporation during long flights. Nevertheless, it has been said that birds produce so much heat during flight that they must use water to cool themselves. It has been calculated that a pigeon flying at 70 km h-1 requires about 8 grams of water for cooling for each gram of fat used as fuel, but this is based on untenable assumptions about the production and dissipation of heat in a flying bird. This becomes obvious if we consider long-distance migrations which may take place over many hundreds, perhaps even thousands, of kilometers, at times over open ocean without any opportunity to feed or drink. (For a recent, well-considered review of the energetics of bird migration, see Tucker, 1971.) When a migrating bird starts on its flight, as much as 50% of the body weight may be fat. If, in addition, it should need to carry several times the weight of the fuel as cooling water, the situation would be patently absurd.