Interspecific variation in avian thermoregulatory patterns and heat dissipation behaviours in a subtropical desert

In hot, arid environments, daily air temperatures can exceed avian normothermic body temperatures, posing a major physiological challenge for desert birds. To maintain stable core body temperature (Tb) and avoid lethal hyperthermia, birds employ physiological and behavioural adjustments that return Tb to a normothermic range, often involving costs related to increased water and/or energy demands. Birds housed in large free-flight aviaries in the Kalahari Desert, each equipped with a surgically-implanted Tb logger, exhibited substantial interspecific variation in Tb patterns at high air temperatures (Tair). While most species showed an increase in Tb with Tair, others displayed no pattern or a negative relationship between Tb and Tair. Most species avoided prolonged hyperthermia, with reductions in activity and increased shade-seeking evident in several study species. Access to drinking water and food resources in captivity may have affected Tb patterns. The study examined nine species representing three orders that vary substantially in their heat dissipation thresholds, specifically pant50, the Tair at which panting behaviour is present in 50% of observations. Birds use behavioural adjustments such as reducing activity and increasing shade-seeking behaviour to reduce physiological costs of thermoregulation at high Tair. However, the conflict between actively foraging to maintain energy and water budgets, and hyperthermia avoidance by reducing activity and seeking shade can be expensive in terms of lost opportunity costs. The study revealed that thermoregulation varies substantially among species, and suggests that free-ranging birds in hot, arid environments may maintain higher Tb than currently thought. The data also indicate that functional links exist between interspecific variation in heat dissipation behaviour (HDB) and Tb regulation. The study's findings have implications for understanding avian responses to climate change and for modelling species vulnerability to high temperatures in hot environments. The study's results are consistent with the notion that thermal performance of endotherms represents a continuum from thermal specialisation to generalisation, akin to that found in ectotherms. The study's findings suggest that species can exhibit a range of thermoregulatory strategies, from thermal specialists that maintain high performance over a narrow Tb range, to thermal generalists that tolerate a wider range of Tb but at lower performance levels. The study's results also highlight the importance of considering interspecific variation in thermoregulatory patterns when assessing species vulnerability to high temperatures in hot environments.