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).