Bird migration on the edge: Experimental manipulation of corticosterone advances departure dates.

Endogenous reserves accumulated during migration stopovers help most migratory birds cope with environmental uncertainties and fuel energy demands associated with migration and/or reproduction. Migratory decisions, such as departure time, should thus be finely tuned to energy intake rate at stopover sites. However, the physiological drivers of these decisions remain poorly understood. Glucocorticoids such as corticosterone (CORT) are known to mediate the stress response in birds but also play a key role in the regulation of energy intake and behavior, particularly during demanding life-history stages. We investigated how baseline CORT influences bird energy acquisition and migratory decisions by manipulating the physiology of wild snow geese (Anser caerulescens atlanticus) through subcutaneous implantation of corticosterone pellets during spring stopover. Birds of similar body condition were paired, implanted with CORT or placebo pellets, and tracked with GPS-GSM collars and accelerometers to monitor foraging efforts, habitat use, and migration departure date. We measured foraging rates from accelerometer data and classified using an unsupervised algorithm calibrated with field video recordings. CORT-treated birds foraged 20% more on average than placebo individuals over 10 days, primarily by increasing foraging efforts rather than altering habitat use. Most of the difference occurred in the first days post-implantation (Foraging rates on Day 2, CORT: 0.4 [95% CI: 0.34, 0.47]; placebo: 0.3 [0.2, 0.36]) and gradually faded to zero afterward (Foraging rates on Day 10, CORT: 0.26 [0.21, 0.32]; placebo: 0.26 [0.21, 0.31]). These higher foraging rates advanced the median departure date of CORT-treated individuals by 2 days compared to placebo (median departure: CORT, May 17 [15, 17]; placebo, May 19 [17, 20]). Our experimental manipulation is one of the first to induce a positive shift in migration phenology and confirms the role of CORT baseline levels in modulating energy acquisition and migratory decisions in a wild bird species.