Blood mitochondrial metabolism predicts flight performance without lasting effects of early-life thyroid hormones

Early-life conditions can shape adult phenotypes by influencing processes linked to energetic homeostasis. Thyroid hormones (THs) enhance mitochondrial electron transport chain capacity (ETS) in fledglings, but whether this effect is permanent and influences fitness-related traits is unclear. We tested whether ETS remains elevated in adult zebra finches treated with TH during development, predicting that higher ETS would be reflected in energetically demanding traits such as exploratory-like behavior and escape flight. Early-life TH did not alter adult erythrocyte mitochondrial aerobic metabolism. Mitochondrial aerobic metabolism decreased from the energetically demanding growth phase to the less demanding adulthood. Independent of the treatment, mitochondrial cellular metabolic rate and oxidative phosphorylation were positively associated with high-intensity escape flight but not with exploratory-like activity. These results do not support early-life TH as a driver of long-term metabolic or behavioral traits but instead show a link between blood mitochondrial metabolism and energetically costly avian flight performance.