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

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.