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46% larger at the peak of winter relative to summer in chickadees, our observations suggest that winter BMR varies relatively independently from changes in the mass of digestive organs in this species. The lack of relationship between body composition and BMR during the summer was also a surprising result but, although we cannot rule out that a decline in tissue metabolic intensity at that time (Zheng et al, 2008; Zheng et al., 2013b) may uncouple relationships between organ mass and BMR, this could also be due to our smali sample size for this period (n = 11).

An organ variable that was found to consistently affect BMR throughout the year was lean dry muscle mass. Indeed, body muscles explained up to 35% of BMR variations in three out of five sets of analyses and were selected as the first variable of importance when using pooled data for all periods. Despite the current interpretation of BMR being a variable mainly reflecting variations in the mass and activity of organs found in the intemal cavity (Bumess et al, 1998; Liu & Li, 2006; Piersma et al, 1996; Piersma & Van Gils, 2010), our findings corroborate previous studies (Chappell et al, 1999; Konarzewski et al., 2000) where a significant relationship was found between BMR and muscle mass. Because BMR reflects the energy consumption of all lean tissues in a resting animal and sińce the lean dry mass of muscles represented 64% of total lean dry body mass in our birds, it appears that the main contributing tissue to BMR variation in chickadees was skeletal muscles. Muscles consume little energy at rest relative to their maximal capacity (Scott & Evans, 1992). However, because they represent a large part of total lean tissue mass, variations in their size may inevitably have a strong influence on BMR.

4.6.4 Variation in organ mass and relationships with Msum

We predicted that winter acclimatization in chickadees would be associated with an enlargement of the exercise organs to improve heat production capacity and cold endurance and that the variation in mass of these organs (muscles and cardiopulmonary) would explain most of the variability in Msum. As expected, Black-capped chickadees expressed their highest level of Msum at midwinter (+12-14% relative to fali) and Msum showed a seasonal pattem remarkably well paralleled by the enlargement of the muscles and cardioplumonary organs (+11% and +14% between fali and midwinter, respectively, compare also figures 4.3b with 4.4b and 4.4c).