Ecological Energetics of Wintering MerlinsFalco columbarius

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Abstract
The ability to maintain a balanced energy budget through extended periods of thermal stress is a major determinant in the survival of birds resident in the north temperate zone during winter. We developed a time-energy budget for winter-acclimated merlins (Falco columbarius) to examine the relationship between energy expenditure and temperature, wind, and radiation and to learn how merlins cope with winter on a physiological and behavioral basis. Nine free-living birds were radio tagged and their activities monitored to establish an activity budget for the winterperiod (November 1 to February 28). The data were combined with thermoregulatory and activity costs of nine other merlins measured in an open-circuit respirometer. Basal metabolic rate was higher in females (6.96 kJ · h⁻¹) than males (5.23 kJ · h⁻¹) and both were more than 50% above level spredicted from their mass. Lower critical temperatures were 3.4° and 14.3°C for females and males, respectively, and thermal conductance in males (0.267 mW · [g · °C]⁻¹) was about 7% lower than predicted from mass, while for females (0.245 mW · [g · °C]⁻¹) it was within 1% of the predicted value. The allocation of time in the activity budget was based on minimizing foraging activity and energy expenditure, rather than maximizing energy intake. Physiological adaptation shown by merlins suggests that biotic rather than physiological factors previously limited merlin abundance on the northern Great Plains in winter.