Metabolic responses at sub-thermoneutral temperatures in captive common waxbills (Estrilda astrild)

Understanding how invasive species cope with novel thermal environments can help clarify the physiological mechanisms supporting establishment and range expansion. Here, we measured resting metabolic rate in captive-bred common waxbills (Estrilda astrild), a successful Afrotropical invader, across ambient temperatures ranging from 11 to 35 °C. We also assessed individual metabolic reaction norms, repeatability, core body temperature and thermal conductance to determine whether birds followed classical thermoregulatory expectations or showed evidence of reduced energetic costs at low temperatures. Metabolic rate showed a non-linear relationship with ambient temperature, with a lower critical temperature at ∼28.9 °C and a second inflection at ∼18.8 °C. Below this second threshold, metabolic rate levelled off rather than increasing linearly as predicted by the Scholander-Irving model. This pattern coincided with lower thermal conductance at colder temperatures and modest reductions in body temperature, suggesting that the deviation cannot be attributed to metabolic downregulation alone. Individuals differed consistently in overall metabolic level, but showed little evidence of consistent differences in reaction-norm slopes. Together, these results suggest that common waxbills may reduce thermoregulatory costs under mild cold exposure through coordinated physiological adjustments, a flexibility that may contribute to their persistence in cooler invaded environments.