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 (Tair) regularly exceeding avian body temperature (Tb). 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 Tair in 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 housed during mid-summer in large free-flight aviaries in the Kalahari Desert each received a surgically-implanted Tb logger, and we quantified shade-seeking, activity and panting behaviours to examine relationships between species-specific pant50 and Tb regulation. Overall, species setpoint Tb values were higher (range: 41.4 ± 0.5 °C to 43.1 ± 0.4 °C) than expected with maximum Tb values of 43.4-45.5 °C. Interspecific variation in Tb patterns at high Tair was substantial, with Tb increasing with Tair in most species, whereas in others no pattern or a negative relationship between Tb and Tair 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 Tb patterns. Our data reveal that thermoregulation varies substantially among species, and suggest that free-ranging birds in hot, arid environments may maintain higher Tb than currently thought.