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Seasonal and experiinental changes in Msum have also been interpreted in terms of body remodelling. Msum is the maximal shivering heat production capacity (Cooper & Swanson, 1994; Swanson, 199la; Vćzina et al, 2006) and studies highlighted parallel increases in thermogenic capacity and pectoral muscle size in free-living (Cooper, 2002; 0'Connor, 1995; Swanson et al., 2013) as well as in captive birds (Vezina et al., 2011; Vćzina et al., 2007). Consequently, it is commonly assumed that variations in Msum reflect changes in the size of skeletal muscles (Cooper, 2002; Saarela & Hohtola, 2003; Swanson et al., 2013; Vezina et al., 2007).

We recently showed in free-living Black-capped chickadees (Poecile atricapillus) that metabolic performance varied within the winter season (Petit et al., 2013). In this study, we found that BMR remained at its summer level in autumn until November and then increased to reach a peak 6% higher in February while Msum had already achieved 20% of its inter-seasonal change by October, attaining in February a level 36% higher than the summer level measured in August. This within-winter variation in metabolic performance suggests that the size and activity of the organ machinery responsible for changes in metabolic rates should also vary throughout winter. However, the uncoupling in the timing of seasonal changes in BMR and Msum implies that intemal organs and skeletal muscles may follow dissimilar paths through time in response to parallel but relatively independent constraints (Petit et al., 2013).

This study follows up on previous work (Petit et al., 2013) and investigates seasonal changes in body composition and metabolic performance of ffee-living black capped chickadees wintering in eastem Canada. Our objectives were (1) to characterize the pattems of organ mass variation throughout the year with a focus on the winter season; (2) to determine the contribution of these organs to BMR and Msum variations and (3) to determine if the effect of specific organs on metabolic ratę variations were constant over the year. We expected that chickadees would cope with the winter increase in thermoregulation and energy reąuirements by enlarging exercise (e.g. muscles, heart, lungs) (Liknes & Swanson, 201 lb) as well as digestive and excretory organs (e.g. intestines, liver, stornach, kidneys) (Williams & Tieleman, 2000) and that this would conseąuently explain the increase in metabolic performance associated with the peak of cold. Morę precisely, based on observations by others, we expected a predominant role of the digestive (e.g. intestines) and excretory (e.g. kidneys) organs in explaining BMR variations (Liu