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the warmest minimal temperaturę never reaches this value. This implies that by the end of August, time at which temperatures are still relatively warm, chickadees are already beginning to improve their thermogenic capacity in preparation for the incoming cold conditions. This interpretation goes in hand with our earlier observations (Petit et al., 2013) where we demonstrated that Msum in chickadees from this population had already reached 22% of its inter-seasonal cold acclimatized level by October. This finding also suggests, assuming that the norm of reaction for mass and sex independent Msum is constant across populations, that chickadees from the northemmost populations, such as those found in Alaska, may never be able to reduce their thermogenic capacity to its lowest level sińce maximal summer ambient temperatures typically remains below 20°C at this latitude (Sharbaugh, 2001). These birds may therefore be forced to maintain a certain level of cold acclimatized phenotype even at the warmest time of the year (Olson & Grubb, 2007).

Chickadees reached their maximal population average residual Msum at a mean minimal temperaturę of -10°C. This value is well above the lowest temperatures recorded during the coldest months of winter at our site (often < -20°C, see figurę 2.4) but matches with the average minimal temperaturę measured at that time (Jan: -11.2°C; Feb: -11.5°C) (figurę 2.4). Therefore, by the time minima! ambient temperaturę averaged around -10°C, birds had reached the level of thermogenic capacity that seems to be required for surviving the winter. This suggests that chickadees’ thermogenic capacity is adjusted to average environmental conditions rather than to acute temperaturę extremes.

The highest level of mass and sex independent Msum measured here could represent a physiological and/or a morphological limit (McKechnie, 2008). For example, pectoral muscles could be at their maximal size preventing further increases in shivering capacity at temperatures below -10°C (Petit & Vezina, 2014a; Swanson et al., 2013). Hence, to face temperaturę colder than -10°C, birds would have to use strategies such as microhabitat selection (Wolf & Walsberg, 1996) or hypothermia (Lewden et al., 2014; Sharbaugh, 2001) to compensate for the lack of endogenous heat production. However, one has to keep in mind that Msum is only an indicator of cold endurance (Swanson, 2001) and the level of thermogenic capacity that birds reach when average minimal temperaturę is around -10°C should be sufficient to support short-term events of temperatures below -10°C. The maximal level of Msum reported here (1.55 W) is similar to