Congratulations to Chris Harris and Christine Madliger for publishing one of Chris’ three MSc chapters examining variation in feather corticosterone as a biomarker of environmental change in Conservation Physiology. Entitled “Temporal overlap and repeatability of feather corticosterone levels: practical considerations for use as a biomarker” the paper tested the prediction that for feather corticosterone to be a valid biomarker of exposure to environmental stress, feathers which share spatial and temporal overlap should show similar levels of this glucocorticoid. Chris’ results show that feathers moulted at the same time across the body do not show any similarity in fCORT levels. Along with these results and other experimental work from his thesis, Chris’ results call into serious question whether fCORT can be used as a reliable biomarker of environmental stress.