Increasingly frequent and intense extreme heat events are making heat-related avian mass mortality events 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 mortality event involving wild birds. We quantified body temperature (Tb), metabolic heat production, and evaporative heat loss at air temperatures (Tair) approaching and exceeding normothermic Tb 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 reductions in evaporative cooling capacity and overall heat tolerance, with maximum tolerable Tair ~2 °C lower (45.7 °C) compared to the dry treatment (47.9 °C). A model of end-of-century exposure for the waxbills reveals that elevated humidity will increase the risks of lethal hyperthermia 3- to 7-fold in some parts of southern Africa.