Warmed soon: Early-life thermal stress elevates glucocorticoids and delays dispersal in a long-lived bird

Environmental stress during development can profoundly affect animal physiology and behavior, with potential long-term consequences for fitness. In birds, corticosterone (CORT) mediates responses to developmental challenges, including thermal stress. However, how exposure to extreme temperatures during development shapes post-fledging behavior in free-living species remains poorly understood. We studied 88 Griffon vulture ( Gyps fulvus ) fledglings from three populations in the Iberian Peninsula over three years to test whether extreme weather during development elevates feather CORT concentrations (CORT f ), and whether this hormonal profile is associated with fledging condition and post-fledging performance. We quantified CORT f using enzyme immunoassays and assessed early dispersal behavior through GPS telemetry. We used temperature anomaly data from local weather stations to characterize thermal stress during four standardized developmental periods. Our results show strong interannual variation in CORT f levels, reflecting broad differences in developmental conditions, and demonstrate that nestlings reared under higher-than-average temperatures during early development (from hatching to 40 days of age) had significantly elevated CORT f levels, particularly those raised in exposed nests. Elevated CORT f levels influenced early post-fledging performance: individuals with higher CORT f dispersed later and exhibited reduced daily movements during the post-fledging period. These findings suggest that thermal stress experienced in the nest can alter the physiological trajectory of developing vultures and affect behavioral transitions critical to survival. Given the increased frequency of extreme temperature events under ongoing climate change, our results highlight the importance of developmental conditions in shaping individual variation and population dynamics in long-lived avian scavengers. • Nestling vultures exposed to heat anomalies show elevated feather CORT levels • Nest microclimate buffers thermal stress and modulates stress hormone response • High CORT during development delays dispersal and reduces movement in fledglings • Heat stress in nestlings emerges as a growing threat to vulture conservation