WINTER METABOLISM OF CONIFEROUS FOREST TITS (PARIDAE) UNDER ARCTIC CONDITIONS - A STUDY WITH DOUBLY LABELED WATER

Daily field metabolic rates (FMR) during mid winter of two species of coniferous forest tits, Siberian Tit Parus cinctus and Willow Tit Parus montanus were measured with the doubly-labeled water (DLW) technique north of the Arctic circle in Sweden. The Siberian Tit is approximately 10% larger than the Willow Tit. Despite this difference in body mass no significant differences in field metabolic rates between the two species were found in our sample. Both species expended 10-20% less energy than predicted by recently derived allometric equations. Northern populations of Willow Tits bad significantly higher FMR and metabolic intensities, M (M = FMR/BMR(p)) than a more southern population. This hints at an intraspecific latitudinal cline in basal metabolic rates (BMR). The two species' predicted metabolic rate at their northern boundary in Scandinavia implies that physiological adaptations to winter conditions of both Siberian and Willow Tits have evolved further to the east within their range where they are exposed to harsher climatic conditions during winter.