Corticosterone implants make stress hyporesponsive birds

In birds, the use of corticosterone (Cort) implants is a frequent tool aimed at simulating systemic elevations of this hormone and studying effects on biological traits (e.g. physiology, morphology, behavior). This manipulation may alter adrenocortical function, potentially changing both baseline (BAS-Cort) and stress-induced (STRESS-Cort) plasma Cort levels. However, implant effects on the latter trait are rarely measured, disregarding downstream consequences of potentially altered stress responses. Here we analyzed the effects of Cort implants on both BAS-Cort and STRESS-Cort in nestling and adult European white storks In addition, we performed a review of 50 studies using Cort implants in birds during the last two decades to contextualize stork results, assess researchers' patterns of use and infer current study biases. High and low doses of Cort implants resulted in a decrease of both BAS-Cort (31-71% below controls) and STRESS-Cort (63-79% below controls) in storks. Our review revealed that BAS-Cort generally increases (72% of experiments) while STRESS-Cort decreases (78% of experiments) following implant treatment in birds. Our results challenge and expand the prevailing assumption that CORT implants increase circulating BAS-Cort levels because: (i) BAS-Cort levels show a quadratic association with implant dose across bird species, and decreased levels may occur at both high and low implant doses, and (ii) Cort implants also decrease STRESS-Cort levels, thus producing stress hyporesponsive phenotypes. It is time to work towards a better understanding of the effects of Cort implants on adrenocortical function, before addressing downstream links to variation in other biological traits.