Weather data
For each nestling, local weather data retrieved from The Norwegian
Meteorological Institute (2018) were averaged over 30 days prior to
sampling (at around 11 days old) in order to reflect average
environmental conditions before and during the nestling growth phase
(see Appendix S1). Due to intercorrelation among weather data variables
(Fig. S1.3), we used principal component analysis (Fig. S1.4 and Table
S1.1) to explore relationships between local weather variables and the
regional daily North Atlantic Oscillation (NAO) index (retrieved from
the National Oceanic and
Atmospheric Administration, 2019) averaged over the same 30-day period
(NAO_30). In Appendix S1, we show that the NAO_30 index is positively
correlated with temperature, humidity and atmospheric pressure and
negatively correlated with wind speed and amount of precipitation in our
study area and therefore might be used as a simple index of overall
harshness of weather conditions during the nestling stage; negative and
positive NAO_30 indexes may
indicate “harsh” and “benign” conditions, respectively (e.g.
Stenseth et al., 2003).