Energy Expenditure in Relation to Flight Speed: What Is the Power of Mass Loss Rate Estimates?

Abstract

The relationship between mass loss rate and chemical power in Eying birds is analysed with regard to water and heat balance. Two models are presented: the first model is applicable to situations where heat loads are moderate, i.e. when heat balance can be achieved by regulating non-evaporative heat loss, and evaporative water loss is minimised. The second model is applicable when heat loads are high, non-evaporative heat loss is maximised, and heat balance has to be achieved by regulating evaporative heat loss. The rates of mass loss of two Thrush Nightingales Luscinia luscinia and one Teal Anas crecca were measured at various Eight speeds in a wind tunnel. Estimates of metabolic water production indicate that the Thrush Nightingales did not dehydrate during experimental Eights. Probably, the Thrush Nightingales maintained heat balance without actively increasing evaporative cooling. The Teal, however, most likely had to resort to evaporative cooling, although it may not have dehydrated. Chemical power was estimated from our mass loss rate data using the minimum evaporation model for the Thrush Nightingales and the evaporative heat regulation model for the Teal. For both Thrush Nightingales and the Teal, the chemical power calculated from our mass loss rate data showed a greater change with speed (more U-shaped curve) than the theoretically predicted chemical power curves based on aerodynamic theory. The minimum power speeds calculated from our data differed little from theoretical predictions but maximum range speeds were drastically different. Mass loss rate could potentially be used to estimate chemical power in flying birds under laboratory conditions where temperature and humidity are controlled. However, the assumptions made in the models and the model predictions need further testing. [KEYWORDS: WIND-TUNNEL, GAS-EXCHANGE, MIGRATING BIRDS, PIGEON FLIGHT, WATER, METABOLISM, CALORIMETRY, ENERGETICS, BALANCE, COST]