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5.6 Discussion

In this study, we investigated the effect of winter metabolism on survival of free-living Black-capped chickadees. We expected a positive and non-linear effect of winter physiological condition on both within winter and among year survivals. Our results showed that within winter survival was indeed positively and non-linearly related to maximal thermogenic capacity. However, survival among years was not dependent on any of the measured physiological parameter. This study is therefore the first to show the benefit of increasing winter thermogenic capacity for survival, a proxy of fitness, in free-living birds.

High winter survival in Black-capped chickadees

Apparent survival probability was high (0.92) within winter and remained constant throughout the season despite the seasonal peak of cold occurring in midwinter (see Petit et al._t 2013). This frnding is consistent with observations in chickadees madę by others (Desrochers et al._t 1988; Egan & Brittingham, 1994), including in food-supplemented populations (Egan & Brittingham, 1994). Long term apparent survival was also high (0.96) and constant among years. Loery et al. (1997) and Schubert et al. (2008) suggested that differences in survival between years are resulting from environmental variability, such as change in food availability or in habitat. Given the lack of year effect on survival, our results therefore imply that our population of Black-capped chickadees faced a relatively stable environment during the three years of the study.

Whether we considered it within or among winters, survival was not related to age. Since the highest rates of mortality in juvenile birds occurs during the nestling and fledgling stages (Perrins, 1965; Smith, 1967; Sullivan, 1989), this fmding suggests that the juvenile chickadees that were captured during winter had presumably passed this critical point and were equally likely to survive than the adults.