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the digestive organs (gizzard+intestines) showed a decline in mass by 38% between the fali and summer, a finding also consistent with observations in Eurasian tree sparrows where the intestines were reported to have their highest mass in the fali relative to winter and summer (Liu & Li, 2006; Zheng et al._t 2008).

Although it remained stable, the mass of the excretory organs nevertheless explained part of the variations in BMR when pooled data were analysed. It was also found to relate significantly with BMR during midwinter. This therefore suggests a significant influence of the mass of the liver and kidneys on maintenance costs at the peak of winter, a finding that is not surprising given the key role played by these organs in maintaining fiiel delivery to tissues and the indirect effects of the kidneys on oxygen delivery. Indeed, in addition to its regulating role in carbohydrate, lipid and protein metabolism, the liver produces bile, which is required for assimilating lipids firom the diet. The liver has also been suggested as a potential thermogenic tissue in birds and mammals (Baconnier et al., 1979; Vezina et al., 2006; Villarin et al., 2003; Zheng et al., 2013a). The kidneys are a major site of erythropoietin production in birds (Jacobson et al., 1957) and therefore plays an indirect role in regulating oxygen delivery by blood cells. Under cold conditions, birds are known to increase their food intake (Vezina et al., 2006), thermogenic capacity (Cooper & Swanson, 1994; Swanson, 199la) and hasmatocrit level (the proportion of red blood celi in total blood) (Petit & Vćzina, 2014a; Swanson, 1990b). Therefore, although it is not possible to distinguish the specific contribution of the liver or the kidneys in our study, the effect of the excretory organs on midwinter BMR could be driven by their role in red blood celi production, food processing and heat production.

Model selection identified the digestive organs as contributors to BMR variations during midwinter. However, this effect was weak at best sińce it was not significant and ranked third in importance after the excretory organs and brain. This finding came as a surprise given the experimental and field observations of enlarged digestive organs in cold acclimated or acclimatized birds that support the increased demand hypothesis (Liknes Sc Swanson, 201 lb; Liu & Li, 2006; Williams & Tieleman, 2000; Zheng et al., 2013a). However, as far as we are aware, current evidences are based on observed parallel changes or simple regressions between digestive organ mass and BMR, with no study presenting multivariate analyses to establish the contribution of these organs in the observed variations in BMR. Although digestive organs were