FASTING ENDURANCE AND COLD RESISTANCE WITHOUT HYPOTHERMIA IN A SMALL PREDATORY BIRD - THE METABOLIC STRATEGY OF TENGMALM OWL, AEGOLIUS-FUNEREUS

The energetic adaptations of non-breeding Tengmalm's owls (Aegolius fu nereus) to temperature and fasting were studied during the birds' autu mnal irruptions in western Finland. Allometric analysis (including lit erature data and two larger owl species measured in this study) indica tes that the basal metabolic rate of owls is below the mean level of n on-passerine birds. However, the basal metabolic rate of the 130-g Ten gmalm's owl (1.13 W) is higher than in other owls of similar size. Thi s is probably related to its northern distribution and nomadic life hi story. Relative to its size, Tengmalm's owl has excellent cold resista nce due to effective insulation (lower critical temperature + 10 degre es C, minimum conductance 0.19 mW.cm(-2).degrees C-1). Radiotelemetric measurements of body temperature showed that the level of body temper ature is lower than for birds in general (39.4 degrees C at zero activ ity) and that the amplitude of the diurnal cycle is also low (0.2-0.6 degrees C). In contrast to many other small birds, Tengmalm's owls do not enter hypothermia during a 5-day fast at thermoneutrality or in co ld. Moreover, while the metabolic rate per bird shows the expected mas s-dependent decrease, the mass-specific rate decreases only slightly d uring the fast. In line with this, there was no decrease in the plasma triiodothyronine concentration during the fast in the owl, whereas a dramatic drop was observed in the pigeon and Japanese quail that were used as a reference. Despite this, the owl has an excellent capacity f or fasting because of its ability to accumulate extensive fat depots a nd its low overall metabolic rate. Fasting reduced evaporative water l oss to 50% of that in the fed state. Calculations show that the oxygen consumption observed in fasting birds would involve a production of m etabolic water barely sufficient to compensate for evaporative water l oss. The threat of dehydration may thus set a limit to the decrease in metabolic rate in fasting owls (owls rely totally on water either ing ested with food or produced metabolically). We conclude that the metab olic strategy in Tengmalm's owl is largely dictated by an evolutionary pressure for fasting endurance. With the restrictions set by small bo dy size and water economy, this bird has apparently taken these adapta tions to an extreme. The constraints that preclude hypothermia, which could increase the capacity for fasting even more, remain unknown.