Functional Ecology 2007 21, 528–533 528 © 2007 The Authors. Journal compilation © 2007 British Ecological Society Blackwell Publishing Ltd Sources of variation in winter basal metabolic rate in the great tit J. BROGGI,*† E. HOHTOLA,* K. KOIVULA,* M. ORELL,* R. L. THOMSON* and J.-Å. NILSSON‡ *Department of Biology, University of Oulu, PO Box 3000, FIN-90014 Oulu, Finland, and ‡Department of Ecology, Animal Ecology, University of Lund, S-223 62 Lund, Sweden Summary 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. Key-words: BMR, parus major, energetics, interpopulation comparison, age Functional Ecology (2007) 21, 528 –533 doi: 10.1111/j.1365-2435.2007.01255.x Introduction Despite the large amount of published data on the basal metabolic rate (BMR) of various species, infor- mation about the factors, apart from body mass, that cause variation in BMR in wild populations is scarce. The few studies that have been performed are mostly restricted to interspecific analyses (McNab 1988; Rezende, Bozinovic & Garland 2004), with intraspe- cific analyses mostly confined to laboratory-reared animals (Ksiazek, Konarzewski & Lapo 2004; Labocha et al . 2004; Sadowska et al . 2005). Recent evidence from some of these studies suggests that intraspecific variation in BMR has a strong genetic component (Wikelski et al . 2003; Ksiazek et al . 2004; Sadowska et al . 2005). However, BMR also appears to be an extremely plastic character, which varies not only between populations but also at an individual level (Bech, Langseth & Gabrielsen 1999; Kerimov & Ivankina 1999; Horak et al . 2002; Labocha et al . 2004; Speakman, Król & Johnson 2004). In addition to genetic and environmental effects, intraspecific variation in BMR may be due to developmental factors or to strategic decisions (Daw- son et al . 1983; Burness, Ydenberg & Hochachka 1998; Swanson & Olmstead 1999; Nilsson 2002; Bech et al . 2002; Wikelski et al . 2003; Broggi et al . 2005). Individuals of endotherm species living year-round at temperate to arctic latitudes are subject to special environmental factors. In order to survive, such indi- viduals have to cope with marked seasonal changes in both food availability and energy requirements for thermoregulation. In winter, both non-renewable food resources and the time available to obtain them decrease, while thermostatic costs increase. Such factors com- bine to make winter an energetically stressful period, especially for day-feeding, arboreal species such as small birds, which undergo a process of winter acclimatiza- tion consisting primarily of a metabolic improvement in thermogenic capacity and endurance (Dawson & O’Connor 1996; Pravosudov & Grubb 1997). How- ever, to fulfil such metabolic requirements birds need †Author to whom correspondence should be addressed. E-mail: Juli.Broggi@oulu.fi