Metabolic response to temperature variation in the great tit:: an interpopulation comparison

1. We studied the resting metabolic rate (MR) from two great tit Parus major (Linnaeus) populations living in different winter regimes. Birds from the two different localities were exposed individually to +25 degreesC, 0 degreesC and -10 degreesC for the night in three consecutive sessions in random order. 2. Birds from Lund (Sweden) had a lower basal MR, as measured at thermoneutrality (+25 degreesC), than had birds from Oulu (Finland). Nevertheless, below thermoneutrality, birds from Oulu spent relatively more energy, especially at -10 degreesC. 3. Although the energy needed for thermoregulation decreased with increasing basal MR this relation is at a higher metabolic cost for birds in Oulu than for birds in Lund. 4. The higher basal MR in Oulu is probably a consequence of a higher maximal MR needed in the severe cold. Further, the observed MRs below thermoneutrality are lower than expected from published data. This suggests that all birds were probably hypothermic at -10 degreesC, particularly Lund birds, and that the use of controlled hypothermia in great tits may be more common than thought previously. Great tits seem to rely primarily on metabolic adjustment to cope with the harsh climatic conditions in the northernmost parts of its distribution.