2.3 Temporal, individual, and environmental variables affecting survival
To quantify seasonal variation in survival, we defined four discrete seasons by date that correspond to differences in environmental conditions and the typical behaviour of juvenile birds: (1) summer (15 May until 1 August): the post-fledging phase after juveniles first leave the nest (previously analysed in Perrig et al. 2017); (2) autumn (2 August – 23 October): the dispersal period when individuals permanently depart from their natal territory, and environmental conditions change to shorter days and cooler temperatures; (3) winter (24 October – 12 March): the period when birds establish and occupy a winter home-range, and endure occasionally cold winter weather during which food can become inaccessible; (4) spring (13 March – 15 June): the first breeding period when birds acquire and occupy the first territory and breed during gradually warming weather with longer day lengths. Note that our season definition includes a deliberate overlap of 4 weeks between subsequent years to accommodate the staggered fledging date of juveniles and staggered onset of first reproduction.
Besides temporal variation, survival may also differ by age, sex, and body mass (Le Gouar et al. 2011; Tschumi et al. 2019). We therefore measured body mass (to the nearest 0.1 g) and tarsus length (to the nearest 0.1 mm) at the day of tagging, and corrected these measurements for the age of the bird at the time of measurement (Perrig et al. 2014). We specified the age of juveniles in days based on standard pictures and the known fledging date (Perrig et al. 2017). In addition, feather samples were obtained for genetic sex determination of the nestlings (Tschumi et al. 2019). In 2010 and 2011 roughly half of the monitored broods were provided with supplementary food during the nestling stage (Perrig et al. 2017), and we recorded whether individuals had benefitted from supplementary feeding or not.
To investigate the effect of winter conditions on survival probabilities, we extracted daily snow depths from the weather station Sachsenheim (Germany, station ID: 04349, downloaded from: https://opendata.dwd.de/climate_environment/CDC/observations_germany/climate/daily/kl/historical/, accessed 16 Oct 2023). Snow depth is known to limit owl’s access to food and is therefore a primary indicator for winter harshness and food availability that would influence survival (Altwegg et al. 2006; Le Gouar et al. 2011; Thorup et al. 2013). We calculated the number of days with a snow cover ≥ 1 cm for each two-week encounter occasion.