AbstractAllostatic load describes the interplay between energetic demand and availability and is highly context dependent, varying between seasons and life‐history stages. When energy demands exceed physiological set points modulated by glucocorticoid hormones, individuals may experience allostatic overload and transition between stages in sub‐optimal physiological states.Corticosterone, the major glucocorticoid hormone regulating energy expenditure in birds, is incorporated into growing feathers (CORTf), and it has been suggested that CORTfreflects long‐term records of allostatic load during feather growth. However, relationships between allostatic load and CORTfhave not been adequately evaluated. If such relationships exist, the use of CORTfto investigate cross‐seasonal effects could provide novel insights into impacts of past allostatic load and/or overload events.We tested whether experimental increases in daily workload during two adjacent life‐history stages would be reflected in CORTflevels, and examined if CORTflevels reflected either current energetic demand or allostatic load prior to feather growth.Daily workloads in female mallardAnas platyrhynchosducklings were increased over a 6‐week period using physical obstacles and/or carrying back‐mounted weights. We measured daily energy expenditure, growth, body mass, and CORTfin growing ducklings. Then, we induced feather moult and reapplied combinations of workload treatments for an additional 6 weeks to investigate whether effects of past energetic demands would be detected in future CORTflevels.Ducklings confronted with higher workloads during development had reduced body mass, growth rates and consequently higher daily energy expenditure and CORTfvalues compared to controls. When ducklings were fully developed, CORTfpatterns in birds re‐exposed to workload treatments reflected only current, rather than past, energetic demands. However, under allostatic overload conditions, past levels of CORTfwere positively associated with CORTfin the subsequent moult.Our study confirms the previously untested assumption that CORTfreflects energetic demand during the period of feather growth in a precocial bird. We show that allostatic overload conditions early in life, which temporarily suppress growth, can be detected using CORTf, an event potentially missed in studies which rely solely on measures of body condition alone.We suggest that CORTfcan provide a valuable biomarker of allostatic load and overload conditions during the period of feather growth, but highlight how context should be considered for studies using CORTfto investigate influences of carryover effects. Our study contributes to building a physiological foundation to inform interpretations of ecological patterns using CORTf.Aplain language summaryis available for this article.