“…Subsequent studies on corticosterone in feathers were performed mostly in wild birds (e.g., Bortolotti et al., 2009 ; Koren et al., 2011 ; Lattin et al., 2011 ; Fairhurst et al., 2012 ; Lendvai et al., 2013 ; Harms et al., 2015 ; Kouwenberg et al., 2016 ; Aharon-Rotman et al., 2017 ; Freeman and Newman, 2018 ; Monclús et al., 2020 ), using feather corticosterone as a retrospective view on challenges the birds had to cope with during feather growth. Despite the wild birds, results of first investigations in poultry ( Berkvens, 2012 ; Carbajal et al., 2014 ; Jenni-Eiermann et al., 2015 ; Zeinstra et al., 2015 ; Johns et al., 2017 ; Weimer et al., 2018 ; Alba et al., 2019 ; von Eugen et al., 2019 ; Nordquist et al., 2020 ; Lindenwald and Rautenschlein, unpublished data) are also encouraging; however, in the absence of a standardized procedure, authors applied different methods to detect and quantify corticosterone in feathers. These variations, such as variations in the amount of feather material or methanol volume for the extraction, crushed vs. grind up feathers, and different methods for filtration or different assays, make it rather impossible to compare the results, properly ( Romero and Fairhurst, 2016 ).…”