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significantly with BMR at midwinter, the effect of the digestive organs on that variable was not significant.

4.6.1 Seasonal yariation in body mass and metabolic performance

Black-capped Chickadees, as other passerines (Lehikoinen, 1987), have been shown to express daily variations in body mass during winter, which are interpreted as changes in the size of endogenous lipid reserves (Mandin & Vezina, 2012). Our results are in agreement with this interpretation as we found a significant effect of time of capture on size-independent body mass but not on size-independent lean dry mass. At the seasonal scalę, although body fat content remained relatively constant from the fali to the end of winter, we found a midwinter peak in both size-independent body mass and size-independent lean dry body mass (figurę 4.2). The variations in lean dry body mass paralleled that observed in muscles and cardiopulmonary organs (compare figures 4.2 and 4.4) and the peak of mass coincided with that of body water content. In contrast with earlier findings suggesting that seasonal winter body mass variations are the sole result of fat accumulation in Black-capped chickadees (Chaplin, 1974), our results suggest that the winter peak in body mass is also resulting from a larger proportion of lean tissues (e.g. skeletal muscles) in the body, which are known to contain morę water relative to fat (Gerson & Guglielmo, 201 la, 201 lb).

As observed in other studies on chickadees and other avian taxa (Cooper & Swanson, 1994; Vćzina et al, 2006; Zheng et al., 2008), birds increased their metabolic performance during winter. Whole BMR culminated at midwinter with values on average 6% higher than during summer while whole Msum was on average 29% higher at midwinter relative to summer. These values are consistent with previous observations conducted at the same time in the same population but using a larger sample size. Indeed, Petit et al (2013) reported a 6% increase in BMR and a 34% increase in Msum between peak of winter and summer in different birds from the same population. In the present case, however, the seasonal changes in BMR were not significant when considering the effects of lean dry body mass. Therefore, as suggested earlier (Petit et al., 2013), it appears that the increase in heat production capacity associated with cold acclimatization does not require an upregulation of maintenance costs in our population. BMR