A heat-sensitive songbird's risk of lethal hyperthermia increases with humidity

Frequent and intense heatwaves are causing heat-related avian mass mortality events to become more common, but the role of elevated humidity as a contributing factor remains unclear. Here, we quantified the effect of humidity on risks of lethal hyperthermia for blue waxbills (Uraeginthus angolensis), the species most common among the victims of South Africa's first documented heat-related mass mortality event involving wild bird populations. We quantified waxbills' body temperature (T_b), metabolic heat production and evaporative heat loss at air temperatures (T_air) approaching and surpassing normothermic T_b in dry (1.1 ± 0.9 g m^-3) and humid (21.3 ± 0.4 g m^-3) air. The humid treatment was associated with significant declines in evaporative cooling capacity, and maximum T_air tolerated by waxbills was approximately 2°C lower (45.7°C) compared to the dry air treatment (47.9°C). A model of end-Century exposure for the waxbills reveals that elevated humidity could increase the risks of lethal hyperthermia by threefold to sevenfold in some parts of southern Africa.