Ecological energetics of the Kestrel:Daily energy expenditure throughout the year based on time-energy budget, food intake and doubly labeled water methods

1. Three methods were employed to determine the daily energy expenditure (DEE) of free-living Kestrels, throughout the year in a study area in the northern Netherlands. - TEB (time budget analysis) - Food intake (observational method) - DLW (doubly labeled water method) 2. The TEB model used here relies on empirically determined cost factors for the behaviour categories distinguished. To obtain average daily energy expenditure the average fasting metabolic rate for birds under thermoneutral conditions as determined in the laboratory is adjusted for thermoregulatory cost under field conditions. The environmental factors impinging on the animal were evaluated by exposing a heated taxidermic mount to ambient conditions. To this value were added the cost factors for activity as well as for tissue synthesis and empirically determined values for the heat increment of feeding. Where appropriate, the increased energetic expenditure associated with moult has also been added. 3. In all, comparison of TEB predictions and DLW measurements is possible for 15 sessions lasting from one to three days. Agreement was good, the time budget model tending to overestimate actual energy expenditure by about 7%. A further 12 DLW determinations, lacking complete time budget data, when compared to the mean, expected by TEB prediction for the period of the year concerned, confirm this (discrepancy about 1%, TEB again overestimating). 4. The estimation of daily expenditure from the observations of prey capture when corrected for body mass changes (D) yields figures lower than the TEB prediction by about 11 %. After excluding episodes of extreme discrepancies, which are probably due to shortcomings of the food intake method under special circumstances, food intake estimated DEE about 6.5% lower than TEB predictions. We conclude that this method is for a large part of the annual cycle essentially in agreement with the DLW value. 5. The seasonal pattern of energy expenditure in the Kestrel as estimated by the TEB approach is given. Males experience an extended annual peak during reproduction when they provision both their mate and progeny, whereas females peak during the late nestling phase when they actively participate in feeding. 6. On account of the energy subsidy provided by the male and lower mass-specific basal metabolic rate, the total annual energy consumption of the heavier female Kestrel is slightly less than that of her mate. Consequently the mass-specific metabolic rate in males is 20% higher than in females, which might have, still unknown, different consequences for the expected longevity of the sexes