Long-term measures of climate unpredictability shape the avian endocrine stress axis

The vertebrate glucocorticoid stress response is an important mechanism
facilitating pleiotropic phenotypic adjustments for coping with
environmental change and optimizing fitness. Although circulating
glucocorticoid hormones are mediators of plasticity that individuals can
adjust rapidly in response to environmental challenges, they are also
shaped by ecological selection. It remains unclear, however, how
environmental variation on different timescales influences
glucocorticoids. Here, we use an intraspecific comparative approach to
determine how variation in precipitation on different timescales (months,
years, decades) shapes distinct components of the glucocorticoid response.
We sampled superb starlings (Lamprotornis superbus) at eight sites across
Kenya in multiple years that differed in precipitation. Among-population
variation in baseline glucocorticoids was shaped by both short- and
long-term precipitation, whereas variation in stress-induced levels was
poorly explained by precipitation on any timescale. Adrenal sensitivity,
quantified via adrenocorticotropic hormone (ACTH) injections, was shaped
by long-term precipitation and highest in unpredictable environments.
Together, these results suggest that variation in glucocorticoids can be
best explained by environmental variation at timescales that extend beyond
the lives of individuals, though baseline glucocorticoids also reflect
short-term environmental conditions. Patterns of long-term precipitation
may represent a microevolutionary selective pressure shaping the endocrine
stress axis across populations and influencing how individuals cope with
environmental change.