Energy expenditure in relation to flight speed: what is the power of mass loss rate estimates?

The relationship between mass loss rate and chemical power in Eying birds i s 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 applicab le when heat loads are high, non-evaporative heat loss is maximised, and he at balance has to be achieved by regulating evaporative heat loss. The rate s of mass loss of two Thrush Nightingales Luscinia luscinia and one Teal An as crecca were measured at various Eight speeds in a wind tunnel. Estimates of metabolic water production indicate that the Thrush Nightingales did no t dehydrate during experimental Eights. Probably, the Thrush Nightingales m aintained 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 Nig htingales and the Teal, the chemical power calculated from our mass loss ra te 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 the oretical predictions but maximum range speeds were drastically different. M ass loss rate could potentially be used to estimate chemical power in flyin g birds under laboratory conditions where temperature and humidity are cont rolled. However, the assumptions made in the models and the model predictio ns need further testing.