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5.3 Introduction

For resident bird species living at northem latitudes, winters are characterized by Iow temperatures, reduced food availability, short foraging days and long nights of fasting (McNamara et al._y 1990; Swanson, 2010). To face these challenging conditions, smali species use several physiological adjustments such as increases in lean body mass (Petit et al._y 2014; Vezina et al., 2006; Williams & Tieleman, 2000) and fat reserves (Cooper, 2002; Petit et al., 2014; Swanson, 199la), elevated basal (BMR) and summit metabolic rates (Msum) (Cooper & Swanson, 1994; Petit et al._y 2013) as well as increased haematocrit (Het) (Petit & Vćzina, 2014a; Swanson, 1990b).

Total body mass (M_b), muscle mass and fat reserves are proxies of individual condition (Norte et al._y 2009). BMR, interpreted here as a measure of physiological maintenance costs, is assumed to vary with changes in size of digestive organs and muscles (Chappell et al._y 1999; Liknes & Swanson, 201 lb; Petit et al._y 2014; Williams & Tieleman, 2000; Zheng et al_y 2008) while Msum, a measure of maximal thermogenic capacity, is mainly related to variations in pectoral muscle size and function (Liknes & Swanson, 201 la; Petit & Vćzina, 2014a; Swanson et al._y 2013). Het, the proportion of red blood cells in total blood, is a measure of maximal oxygen carrying capacity. It is related to overall metabolic activity (Carpenter, 1975; Swanson, 1990b) and thus varies positively with BMR and Msum (Bumess et al._y 1998; Petit & Vezina, 2014a). Conseąuently, birds with higher lean mass, larger fat reserves and higher BMR, Msum and Het are assumed to be best eąuipped to perform in a cold environment and to have better survival relatively to birds with lower physiological condition (Brodin, 2007; Pravosudov & Lucas, 2001; Verhulst et al._y 2004). However, although a positive relationship between these parameters and winter survival is commonly assumed (Nzama et al._y 2010; Petit et al., 2013; Sears et al._y 2006; Swanson & Olmstead, 1999) and has been observed in some smali mammals (Boratyński et al., 2010; Jackson et al._y 2001), it has yet to be demonstrated in free-living birds (Swanson & Vezina, 2015).

In this study, we applied a capture-marking-recapture method to track 231 free-living Black-capped chickadees (Poecile atricapilluś) wintering in eastem Canada from the winter of 2009-10 to the winter of 2011-12. We used measurements of Mb, body size, BMR, Msum and Het together with recapture and resighting data to investigate, for the first time in birds,