Energy Expenditure by Free-Living Dippers (Cinclus cinclus) in Winter

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Abstract
The daily energy expenditure (DEE) of free-living dippers (C. cinclus) during winter was measured using the doubly-labeled water (DLW) technique. DEE was estimated as 206 kJ day-1 by this method, compared to 194 kJ day-1 using time-activity bugets combined with laboratory estimates of metabolic costs. The latter result was most sensitive to the assumed costs of underwater-feeding (diving); taking lower and higher costs for this activity resulted in DEE equaling 177 and 212 kJ day-1, respectively. Correlation between individual estimates of DEE obtained using DLW and time-energy bugets was significant, but weak. Of 26 independent variables examined for correlation with DEE and average daily metabolic rate measured using DLW, significant results were limited to various measures of body-size and activity. None of the measured weather variables was significant. For any individual bird, variation in DEE in winter was largely a consequence of changes in the nature and duration of energy-costly activities. Accurate prediction of mean energy expenditure and its confidence intervals using time-budget techniques, at least in species which show a range of activities with differing costs, will depend on precise evaluation of those activity costs and their variability.