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.