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5.4.3 Capture-Mark-Recapture (CMR) method
Each bird was marked at capture and released in the population after metabolic measurements. Besides capture sessions, to maximize our probability of resighting marked individuals, we also carried out an average of 1.9 ± 0.1 observation sessions of one hour per month on each site over the whole period of the study. We used both capture and observation from a distance as encounter occasions to calculate the return ratę of birds, which depends on their probability of 1) surviving and coming back to the sampling site (O, the apparent survival probability) and 2) being encountered (p, the encounter probability). Using this protocol, birds from both cohorts were caught and resighted during the winter of their first capture (encounter occasions per bird within the first winter: cohort 1 = 2.5 ± 0.1; cohort 2 = 2.0 ±0.2) and, for cohort 1, also during the following two winters (total encounter occasions per bird of cohort 1 = 4.1 ± 0.2). CMR protocol allowed us to record the encounter history of each bird. For example, an encounter history of “101” meant that a bird had been caught at the first encounter occasion, missed during the second occasion and resighted during the third occasion. To calculate short-term survival (i.e. within winter survival), we used CMR data collected for both cohorts during the winter of their first capture (i.e. winter 2009/2010 for cohort 1 and winter 2010/2011 for cohort 2) while we calculated long-term survival (i.e. among year survival) using CMR data of the cohort 1 collected from Sep 2009 to Dec 2011.
5.4.4 Statistical analysis 5.4.4.1 Data [ paration
Total body mass and size-independent body mass (M,) are expected to be related to individual condition (Norte et al.t 2009). However, because they were highly correlated (r = 0.80), we could not include both variables in the analyses and therefore used only Ms as a measure of condition. Besides a seasonal increase in lean mass and fat reserves (Petit et al.t 2013, 2014), wintering Black-capped chickadees also follow a daily fattening cycle during which they storę fat from sunrise to sunset, which is depleted during the night (Lehikoinen, 1987; Mandin & Vćzina, 2012). Conseąuently, Ms in our birds depended on the time they were caught both during the winter and during the day. Hence, to calculate M„ we first performed a principal component analysis on morphological data (length of head plus beak,
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