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.