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4.3 Introduction

Smali bird species wintering at northem latitudes routinely face energetic challenges owing to cold ambient temperatures (Cooper & Swanson, 1994; Liknes & Swanson, 1996) and restricted access to food (McNamara & Buchanan, 2005; McNamara et al., 1990; Swanson, 2010). This leads to an array of behavioural and physiological adjustments to improve energy acąuisition and thermogenic capacity (Cooper & Swanson, 1994; Swanson, 199la; Swanson & Thomas, 2007; Vezina et al., 2011). In parallel to these changes, adjustments in metabolic performance arc often visible with birds typically expressing higher levels of basal metabolic ratę (BMR, seen as the body’s physiological maintenance cost) and summit metabolic ratę (Msum, an indicator of cold tolerance) in winter relative to summer (Cooper & Swanson, 1994; Swanson, 199la; Vezina et al., 2006; Zheng et al., 2008).

Given the generał interpretation that variations in metabolic parameters, such as BMR, reflect changes in size and activity of body components (Bumess et al., 1998; Chappell et al., 1999; Daan et al., 1990; Piersma & Van Gils, 2010; Weber & Piersma, 1996), BMR variations in response to cold are often perceived as the result of body remodelling (Petit et al., 2013; Swanson, 2010; Vćzina et al., 2006). For example, Williams and Tieleman (2000) showed that captive cold-acclimated hoopoe larks (Alaemon alaudipes) increased their BMR along with the mass of their stornach, intestines, liver and kidneys while Maldonado et al (2009) observed a concomitant increase in BMR and the masses of the liver, kidneys and heart in captive cold-acclimated rufous-collared sparrows (Zonotrichia capensis). Similar results have also been observed in free-living species when comparing summer a winter phenotypes. Indeed, in Eurasian tree sparrows (Passer montanus), Zheng et al (2008) found a higher BMR and heavier intestines, liver and heart in individuals captured in winter compared to those captured in summer. A similar finding was obtained by Liu and Li (2006) in the same species where the elevated winter BMR was shown, in independent correlation analyses, to be positively related to the mass of the gizzard, intestines, liver and heart. Although there are differences in which particular organs covaiy with BMR, these observations suggest a predominant contribution of the digestive organs and heart to the winter increase in physiological maintenance costs (Swanson, 2010).