Effects of Manipulated Incubation Temperature on Baseline Corticosterone Levels in Eastern Bluebirds (Sialia sialis)

Recent climate trends have increased the frequency and severity of aberrant weather. Extreme weather could disrupt homeostasis and negatively impact the likelihood that female songbirds complete the energetically expensive incubation stage. Indeed, historical data demonstrate that cold snaps and heat waves can cause females to abandon nests during incubation. Glucocorticoids are metabolic hormones that orchestrate plastic responses to environmental stimuli including temperature. Under extreme temperatures, baseline corticosterone levels help maintain energy balance so that birds continue with reproduction. We manipulated nest temperatures during incubation in a Southern Appalachian population of eastern bluebirds. Females were randomly assigned to either cold, control, or heat treatments for 7 days. Next, we took a maternal blood sample to measure circulating baseline corticosterone levels. We also measured body condition and plumage color- traits associated with female reproductive effort and success in this species. Although hatching success did not differ with treatment, the heated clutches took longer to hatch- especially later in the breeding season. Yet, nestlings from heated clutches were also slightly more likely to fledge the nest. We expect females will have higher corticosterone after early-season cold treatments and late-season heat treatments. We also explore whether measures of female quality interact with treatment to influence corticosterone levels; high-quality females may be less likely to elevate corticosterone in response to temperature stress.