Seventy years after the discovery that steroid metabolites measured in urine or faeces reflect circulating hormone levels, non-invasive measurements of glucocorticoids and their derivative from various biological matrices have become one of the most essential tools in the functional ecologist's repertoire (Palme, 2019; Sheriff, Dantzer, Delehanty, Palme, & Boonstra, 2011). Glucocorticoids (cortisol and corticosterone) are vertebrate metabolic hormones involved in energy maintenance and regulation whose secretion increases drastically in response to various physiological and psychological stressors. Although the release of glucocorticoids in response to a stressor aimed at maintaining or restoring homoeostasis and provides direct fitness benefits in the short-term, chronically elevated glucocorticoid levels can affect key body functions such as immunity, growth or reproduction, even if the intensity of these effects seems to vary from one species to another (Boonstra, 2013).