Energy metabolism and body temperature of barn owls fasting in the cold

Energetic adaptation to fasting in the cold has been investigated in a noct urnal raptor, the barn owl (Tyto alba), during winter. Metabolic rate and b ody temperature (T-b) were monitored in captive birds, (1) after acute expo sure to different ambient temperatures (T-a), and (2) during a prolonged fa st in the cold (4 degrees C), to take into account the three characteristic phases of body fuel utilization that occur during a long-term but reversib le fast. In postabsorptive birds, metabolic rate in the thermoneutral zone was 4.1 +/- 0.1 W kg-L and increased linearly below a lower critical temper ature of 23 degrees C. Metabolic rate was 70% above basal at +4 degrees C T -a. Wet thermal conductance was 0.22 W kg(-1)degrees C-1. During fasting in the cold, the mass-specific resting metabolic rate decreased by 16% during the first day (phase Ii and remained constant thereafter. The amplitude of the daily rhythm in T-b was only moderately increased during phase II, wit h a slight lowering (0.6 degrees C) in minimal diurnal T-b, but rose marked ly in phase III with a larger drop (1.4 degrees C) in minimal diurnal T-b. Refeeding the birds ended phase III and reversed the observed changes. Thes e results indicate that diurnal hypothermia may be used in long-term fastin g barn owls and could be triggered by a threshold of body lipid depletion, according to the shift from lipid to protein fuel metabolism occurring at h e phase II/phase III transition. The high cost of regulatory thermogenesis and the limited use of hypothermia during fasting may contribute to the hig h mortality of barn owls during winter.