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

Deserts are physiologically challenging environments for birds, with scarce, unpredictable water resources combined with air temperatures (T_air) regularly exceeding avian body temperature (T_b). For arid-zone birds, mismatches between water supply and demand are a constant threat, yet interspecific variation in trade-offs between hyperthermia avoidance and dehydration avoidance remain poorly understood, particularly for free-ranging individuals. We examined behavioural and physiological responses to high T_air in nine species representing three orders that vary substantially in their heat dissipation thresholds, specifically pant₅₀, the T_air at which panting behaviour is present in 50% of observations. Birds housed during mid-summer in large free-flight aviaries in the Kalahari Desert each received a surgically-implanted T_b logger, and we quantified shade-seeking, activity and panting behaviours to examine relationships between species-specific pant₅₀ and T_b regulation. Overall, species setpoint T_b values were higher (range: 41.4 ± 0.5 °C to 43.1 ± 0.4 °C) than expected with maximum T_b values of 43.4-45.5 °C. Interspecific variation in T_b patterns at high T_air was substantial, with T_b increasing with T_air in most species, whereas in others no pattern or a negative relationship between T_b and T_air was evident. Most species avoided prolonged hyperthermia, with reductions in activity and increased shade-seeking evidently adequate to manage heat load without resorting to hyperthermia in in several of our study species. Access to drinking water and food resources in captivity may have affected T_b patterns. Our data reveal that thermoregulation varies substantially among species, and suggest that free-ranging birds in hot, arid environments may maintain higher T_b than currently thought.