Environmentally-induced differences in endocrine systems are among the underlying causes of the phenotypic plasticity observed in many traits. Glucocorticoid (GC) hormones (i.e. corticosterone, cortisol) are involved in the capacity of vertebrates to face environmental changes. This may include immediate adjustments (i.e. “stress response” to a sudden threat) but also long-term phenotypic changes (i.e. lifelong effects of early life conditions). Over the past years, much research has attempted to use GC concentrations as indicators of individual or population welfare or fitness prospects. In the meantime, causes of the wide variability in GC levels among and within individuals remain poorly understood. My research focusses on identifying environmental and internal factors affecting glucocorticoid variation in the short and in the long term. To investigate this, I quantified GC traits in adult zebra finches exposed to benign vs. harsh environmental conditions during development and in adulthood. In this talk I will show the most relevant findings on environmental sources of glucocorticoid variation in our population. I will further provide insights on two functional mechanisms arising as potential mediators of such environmental effects at different scales: metabolic rate and epigenetic processes.