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winter survival and maximal thermogenic capacity was less nuanced. In this case, the cutoff value for survival was 1.35 W. Below this point, birds had no chance of survival (15% of our population) and above this point birds were expected to survive the winter season (85% of the population, figurę 5.2b). In previous studies on the same chickadee population (Petit et al., 2013, 2014), we found that summer (August) levels of raw Msum were on average 1.26 W (unpublished data), a value that should confer to wintering individuals less than 50% chance of survival when considering the effect of body mass and datę on Msum. We also showed that Msum begins to increase before October (Petit et al., 2013) and perhaps even as early as the end of August (Petit & Vezina, 2014b) in this species. Therefore, not only these birds apparently have to increase their thermogenic capacity well ahead of the peak of winter cold but it also seems that individuals that are either too slow or unable to adjust their phenotype from summer to winter have less than 50% chance of surviving. As temperaturę is the main driver of Msum variations in this and other species (Petit & Vćzina, 2014b; Swanson et al., 2014b; Swanson & Olmstead, 1999), it is likely that thermogenic capacity begins to increase as soon as the birds encounter the first cold nights of the early autumn (Petit et al., 2013). The actual level of heat loss reąuired to trigger this upregulation remains, however, to be investigated. Our data therefore confirm for the first time in birds (Boratyński et al., 2010; Jackson et al., 2001) that increasing thermogenic capacity does improve winter survival in small-bodied species.

Why mainlain a higher Msum than needed for survival?

We predicted that chickadees would express maximal survival probability at intermediate Msum levels. However, although the relationship between survival and thermogenic capacity was positive and non-linear, apparent survival was not compromised when Msum reached its highest level. Indeed, our data suggest that winter Msum could be maintained at average levels well above 1.35 W (90% survival) or even above 1.46 W, the threshold value associated with 100% survival (figurę 5.2). Accordingly, in a separate group of birds measured in February 2011, average level of Msum was 1.58 W (based on raw data from Petit et al. 2014 for comparison with figurę 5.2). That is 8 % higher than the heat production capacity seemingly guaranteeing maximal within-winter survival in our population. One can



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