2.2 Monitoring survival
From 2009 to 2011, 93 broods were closely monitored from egg laying
until fledging by conducting regular brood controls and using camera
traps (Perrig et al. 2014; Perrig et al. 2017). Prior to fledging, at an
average age of 28.7 days (± 2.93 standard deviation), all 307 chicks of
these broods were tagged with VHF radio transmitters (Naef-Daenzer et
al. 2005) using backpack harnesses. The total tag mass (including
harness) was 6.9 – 7.2 g (3-5% of the birds’ body mass), the expected
life span of the battery was 400 days and the detection range of the VHF
signals was up to 40 km (for details see Perrig et al. 2017).
We used hand-held antennas to locate all individuals at least three
times per week throughout the study period from May 2009 until May 2012,
except during four two-week intervals in winter 2009 and early spring
2010 when no radio-tracking could be carried out. In an additional six
two-week intervals (mid-winter 2009 and late winter 2010) the tracking
effort was reduced, and individuals were located less than three times
per week.