WINTER STARVATION IN CAPTIVE COMMON BARN-OWLS - PHYSIOLOGICAL STATES AND REVERSIBLE LIMITS

Among birds, the Common Barn-Owl (Tyto alba) is considered to be particularly sensitive to winter-induced starvation. Yet, there are no detailed data on the metabolic response of this species to long-term food deprivation in the cold. Therefore, eight captive Common Barn-Owls (T. a. alba), including both males and females, were fasted at 5-degrees-C ambient temperature, until there was a clear increase in the rate of body mass loss. In wild birds, which fast spontaneously, such an increase (reflecting increased protein utilization) is associated with a triggering of refeeding that anticipates a lethal depletion in body fuels. Weighing the barn-owls every 8 h and collecting excreta for 24-h periods, we found that even after only 0.7 day of starvation, body-mass and nitrogen loss reached low and constant values (at 8.6 +/- SD of 1.0 and 0.17 +/- 0.01 g/day, respectively), which were maintained for 7.2 +/- 1.6 days. This was calculated to correspond to energy equivalents of body-mass loss and daily energy expenditure of 24.3 +/- 1.9 kJ/g and 213 +/- 22 kj/day, respectively. Based on these data, the contribution of proteins to energy expenditure was as low as 8.7 +/- 1.6%. In contrast to what could be expected, when compared to the prefasting level, mean daily energy expenditure per unit body mass was not significantly reduced. Presumably, this was due to the maintenance of a high locomotor activity during the first part of the night, as revealed by a rate in body mass loss two-fold higher than during the light phase. The fast was stopped after 1.3 +/- 0.1 days of increased body-mass loss. The shift to an increased protein utilization was indicated by a 3.5-fold rise in nitrogen excretion associated with a rise in plasma uric-acid concentration from 0.34 +/- 0.08 mmol/L up to 1.75 +/- 0.13, and a drop of plasma free fatty acids from 1.3 +/- 0.3 mmol/L to 0.11 +/- 0.10. Despite the initially heavier females, this metabolic shift occurred simultaneously after 7.9 +/- 1.7 days of starvation for both sexes. Accordingly, at the time of refeeding, the females weighed 240.0 +/- 5.0 g and the males 217.8 +/- 7.2 g (i.e. the initial difference in body mass was maintained). At this time, the barn-owls were still able to fly and refeed by themselves. However, based on data for wild Common Barn-Owls that presumably died from starvation, it can be estimated that the rise in nitrogen loss precedes death by less than 2.5 days.