Sources of variation in winter basal metabolic rate in the great tit

1. Basal metabolic rate (BMR) is the most widely used standard measurement of the cost of living. Despite the acknowledged phenotypic flexibility of BMR, little is known about the patterns of variation in wild animal populations. 2. We studied the sources of variation in BMR of great tit Parus major (L.) among individuals from two wild populations: Oulu (northern Finland) and Lund (southern Sweden) during six consecutive years. 3. By means of a multivariate approach, we found year, locality, date, previous week average minimum temperature, age, body mass, and the interaction between locality and year were the factors retained in the final model, together explaining 71.1% of the total variation in BMR. 4. Birds from Oulu (n = 168) had a higher BMR than Lund birds (n = 156), and their BMR varied more between years than that of Lund birds. The two populations reacted in the same way to the other sources of variation examined. 5. Great tits from both populations showed a positive relationship between BMR and body mass and a negative relationship between BMR and date, previous week average minimum temperature and age. 6. This study highlights the need to standardize BMR measurements when testing predictions about metabolic rates from individuals of wild populations.