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1.3 Introduction

With global changes comes a higher freąuency of unpredictable weather events (Easterling et al., 2000). These short-term environmental fluctuations can affect animal species through changes in demography, phenology (Forcada et al., 2008) and genetic variability (Canale & Henry, 2010), for example by disturbing the timing of reproduction (Laaksonen et al., 2006; Van Der Jeugd et al., 2009) or by favouring genotypes that produce flexible rather than stable phenotypes (Canale & Henry, 2010). This may be particularly important for northem latitude species where waiming is accelerated relative to lower latitudes (EPCC, 2007), especially in winter (DesJarlais et al., 2010), and where the occurrence and amplitudę of short term stochastic events are predicted to increase (EPCC, 2007). Phenotypic flexibility, the rapid and reversible transformations of phenotypic traits that allow adult individuals to adjust their behaviour and physiology to predictable or stochastic changes in the environment (Piersma & Drent, 2003), should provide a certain capacity to buffer these variations (Canale & Henry, 2010). However, in natural settings, little is known on how animals adjust their phenotype to intra-seasonal changes in ecological conditions (McKechnie, 2008; Swanson & Olmstead, 1999).

Winter at northem latitudes is typically considered a challenging season for resident bird species (Cooper, 2000). Since they remain active throughout the cold season, Iow ambient temperatures force these animals to increase energy expenditure for thermoregulation (Cooper & Swanson, 1994; Liknes & Swanson, 1996) while short days, snów and ice cover may reduce foraging time and food availability (McNamara et al., 1990; Swanson, 2010). In smali bodied species, thermoregulatory constraints are exacerbated because of their large surface area relative to volume, which increases heat loss (McNab, 1971), in addition to their limited ability to carry thick insulative plumage. Smali birds therefore use physiological adjustments to improve cold tolerance (Cooper & Swanson, 1994; Swanson, 199la) and their chances of survival. Seasonal acclimatization is typically associated with a winter increase in metabolism visible in parameters such as basal metabolic ratę (BMR; physiological maintenance cost) and summit metabolic ratę (Msum; maximal thermogenic capacity) (Cooper & Swanson, 1994; Swanson, 199la; Zheng et al., 2008). However, although cold acclimatization has been investigated for decades (Scholander et al., 1950c), most field studies lack the required temporal resolution to address questions regarding individual physiological adjustments in response to intra-seasonal variations