1254394686

89

3.6 Discussion

The seasonal inerease in thermogenic capacity commonly reported for wintering birds is often interpreted as the result of an enlargement of shivering muscles (Cooper, 2002; Saarela & Hohtola, 2003; Vćzina et al, 2011; Vćzina et al, 2007). This can be associated with an elevated haematocrit (Swanson, 1990b) interpreted as an upregulation of oxygen carrying capacity. However, evidences linking muscle size to Msum remain correlative (Cooper, 2002; Swanson et al., 2013; Vezina et al, 2006) and the link between haematocrit and Msum has not been demonstrated. In this study, we experimentally enlargcd pectoral muscle size by clipping flight feathers and observed (1) that birds with larger pectoral muscles, including a high proportion of clipped individuals, also had high Msum and (2) that for a given body mass and muscle size, hasmatocrit was indeed positively related to Msum. Results of our phenotypic manipulation therefore support previous correlative evidences for the role of modulating pectoral muscle size and oxygen carrying capacity in Msum adjustments.

On average, birds from all treatments inereased their body mass, the size of their pectoral muscles, their Msum and their haematocrit throughout the experiment These seasonal changes are part of the natural acclimatization response found in wintering Black-capped chickadees. Indeed, both Sharbaugh (2001) and Liknes & Swanson (201 lb) showed inereases in body mass in wintering chickadees relative to summer counterparts. This species is also known to improve its metabolic performance in winter relative to summer (Cooper & Swanson, 1994; Lewden et al., 2012; Petit et al., 2013). For instance, using birds ffom the same population, Petit et al. (2013) reported that chickadees elevated their winter Msum by 25% between the beginning (October) and the peak of winter (February). The observed development of pectoral muscle size is also part of the natural change in phenotype associated with winter acclimatization and has been reported in several species (Cooper, 2002; Liknes & Swanson, 201 lb; 0'Connor, 1995; Swanson, 1991b), including Black-capped chickadees (Liknes & Swanson, 201 lb).

In contrast with control birds that showed an inerease in body mass during the experiment, clipped individuals maintained theirs constant. They also had larger pectoral muscles than control birds, especially during the after period. Therefore, our feather manipulation not only succeeded in forcing clipped individuals to develop larger pectoral muscles, but it also appears