An Evaluation of Time-Budget Estimates of Daily Energy Expenditure in Birds

Abstract

Abstract We used the doubly-labeled water (DLW) technique to measure the daily energy expenditure (ḢTD) of aviary-housed Loggerhead Shrikes (Lanius ludovicianus). Simultaneously to our DLW measurements, we obtained a continuous 24-h record of the bird's time budget (TB) and assessed its thermal environment at 10-min intervals with an array of 23 meteorological sensors that measured the air temperature (Ta), operative temperature (Te), and wind speed (u) experienced by the bird. From the TB and meteorological data, we estimated the birds' ḢTV by several TB models that differed in the energy equivalents assigned to behaviors and in how thermoregulatory costs were calculated. Only a convection-adjusted, electrical-analog model provided a mean ḢTV estimate that was identical to the mean DLW value (106 kJ/day). Values of $\dot{H}_{TD}$ for individual birds calculated by this model ranged from -8.1 to +7.5% of the DLW values and were significantly correlated with the DLW values, indicating that this method accurately gauged the ḢTV of individual birds. Our analysis showed that this model's accuracy resulted from (1) using Te and u to calculate thermoregulatory costs through heat transfer theory, and (2) using measured energy equivalents for the various behavior categories. ḢTV estimates based on other commonly used TB models differed significantly from the DLW values, with mean errors ranging from -18 to +21%.