Great tits differ in glucocorticoid plasticity in response to spring temperature

Fluctuations in environmental temperature affect energy metabolism, stimulating the expression of phenotypic plasticity in behavioral and physiological traits. Vertebrate hormonal signals like glucocorticoids underpin environmentally-induced phenotypic plasticity, with changes in circulating concentrations orchestrating plastic changes in diverse traits. Climate change is predicted to alter temperature variation globally, making it imperative to evaluate whether free-living animal populations can cope adaptively. To evaluate their potential to respond to ongoing global change, we quantified individual variation in glucocorticoid plasticity to ambient temperature in wild great tits ( Parus major ). Using a reaction norm approach, we repeatedly sampled individuals for circulating glucocorticoid concentrations across five years. As expected, baseline and stress-induced glucocorticoid concentrations increased with lower ambient temperatures at the population and within-individual level. Moreover, we provide unique evidence that free-living individuals differ significantly in their plastic responses to temperature variation for both glucocorticoid traits, with some displaying greater plasticity than others. Average concentrations and degree of plasticity covaried for baseline glucocorticoids, indicating that these two reaction norm components are linked. Hence, individual variation in glucocorticoid plasticity to an important environmental factor exists in a wild population, representing a crucial step to assess the adaptive potential of vertebrates to endure current temperature fluctuations.