Individual variation in glucocorticoid plasticity to temperature is not associated with reproductive success in a wild songbird population

Vertebrates, including songbirds, can adapt their behavior and physiology in response to sudden temperature changes through phenotypic plasticity. This adaptability is partly driven by shifts in glucocorticoid levels, but the connection between individual differences in this hormone plasticity and reproductive success has not been established.

Researchers studied the relationship between variation in glucocorticoid plasticity and the breeding success of a wild great tit (Parus major) population over a nine-year period. They collected data on baseline and stress-induced corticosterone levels every time the birds reared their young. Previous research provided evidence for consistent patterns in hormone responses to fluctuating temperatures, indicating individual variations in hormone levels (intercepts) and the strength of the plastic response (slopes), which could potentially lead to evolutionary changes.

However, the study found no link between these hormone variations and reproductive success, measured by the number of fledglings produced or the mass of the brood. This suggests that individual variation in endocrine plasticity may not be subject to natural selection under the current environmental conditions. The findings provide the first empirical evidence of the evolutionary implications of endocrine plasticity in the wild, and emphasize the necessity of assessing its fitness relevance in more extreme conditions, such as those projected due to future climate shifts.