Winter energetics of coniferous forest tits Paridae in the north: the implications of body size

The daily field metabolic rates (FMR) during winter of 3 species of coniferous forest tits, the willow tit Parus montanus Conrad, the crested tit Parus cristatus L. and the coal tit Parus ater L. were measured with the doubly-labelled water technique in the vicinity of Uppsala, central Sweden. FMR-values (kj/day) did not differ between species in spite of the 20% difference in body mass between the coal tit (9g) and the other two species (11-11.5 g). As a consequence, coal tits had significantly higher metabolic intensities (multiples of basal metabolism) than the larger species. Coal tits tended to fly more frequently while foraging than the other species. Maintenance (basal metabolism plus thermoregulation) accounts for 90% of FMR for the willow tit (according to published laboratory measurements), which is the highest value yet reported for any bird species. The coal tit expends as much energy as predicted by allometric equations, while the other two species have surprisingly low field metabolic rates for their body size. The pattern found may explain the migratory tendencies and more southerly distribution of the coal tit. Energy-saving mechanisms may be more important than mere consequences of body size to explain the northern distribution of small wintering birds.