1254394791

XXIII

ABSTRACT

Global changes are associated with an increase in weather stochasticity, which could affect phenology, demography and genetic variability of species. Phenotypic flexibility, which allows individuals to adjust their physiology rapidly to variations of their habitat should provide a certain capacity to buffer environmental fluctuations. In this context, it is therefore crucial to study response capacity of individuals to surrounding variations in order to understand the effect of environmental changes on population dynamics. However, as far as we know, there is a lack of study on capacity for short-term adjustments of physiological parameters in free-living endotherms. With this thesis, we used a population of Black-capped chickadees (Poecile atricapilluś) living in Quebec to study winter adjustments of basal metabolic ratę (BMR) and maximal thermogenic capacity (Msum) in natural conditions. Morę specifically, the objectives of this thesis were to highlight 1) the intra-seasonal pattem of BMR and Msum adjustments (chapter /), 2) the relationship between weather conditions and metabolic performance (chapter 2), 3) the effect of body composition on BMR and Msum (chapters 3 and 4) and 4) the link between winter phenotype and fitness (chapter 5).

With chapter 1, we showed for the first time in natural conditions that this smali avian species is able to adjust its winter metabolism on a short time scalę. Results revealed a winter increase in BMR (6%) and Msum (34%) and a temporal mismatch between these parameters. While BMR began its seasonal increase in November and came back at a summer-like value as soon as March, Msum began to increase at the end of summer and stayed high in March. This difference in the timing of variation in winter BMR and Msum suggests that the two parameters respond to different winter constraints. With chapter 2, we described, for the first time in a ffee-living bird species, reaction norms of BMR and Msum across the natural rangę of weather variations. Results showed that BMR varied between individuals and was weakly and negatively related to minimal temperaturę. Data revealed that Black-capped chickadees adjusted their Msum with minimal temperaturę following a sigmoid curve, increasing linearly between 24°C and -10°C, and with absolute humidity following a U-shape relationship. These results suggest that weather conditions have minimal effects on maintenance costs, which would mainly be individual-dependent. In contrast, thermogenic capacity would mainly respond to weather conditions, especially temperaturę. With chapter 5, using a feather clipping protocol to manipulate muscle size, we demonstrated, for the first time in natural conditions, the causal effect of pectoral muscle size on maximal thermogenic capacity. Results also highlighted that Msum is positively correlated with haematocrit. These findings therefore demonstrate that in a small-bodied ffee-living bird, shivering thermogenesis is supported by pectoral muscle size and probably also by oxygen carrying capacity. In Chapter 4, we studied the relationship between changes in body composition and metabolic performance over the year. Results showed that 64% of Msum variations throughout the year were explained by changes in body muscles and cardiopulmonary organs while 35% of the annual BMR variability was related to variations in mass of body muscles and excretory organs (liveiH-kidney). Finally, results of chapter 5 showed that, in a smali resident passerine, within winter survival was related to Msum following a sigmoid relationship while long-term survival was not dependent on winter phenotype. This study