High humidity significantly amplifies the threat of lethal hyperthermia in small birds on hot days, robbing them of the ability to cool down through evaporation.
The findings are the result of a collaborative study by the University of Cape Town (UCT), University of Pretoria (UP), Wits University and the South African National Biodiversity Institute.
The study, A heat-sensitive songbird’s risk of lethal hyperthermia increases with humidity, was published in the Royal Society Journal Biology Letters.
Co-author associate professor Susan Cunningham, director of UCT’s FitzPatrick Institute of African Ornithology, said the findings raised serious concerns for the survival of species in hot, humid regions undergoing climate change. She emphasised the importance of South Africa’s leading research institutions collaborating to understand and address climate risks to biodiversity.
“These findings highlight that the impacts of climate change on wildlife are not just about rising temperatures, but also about how heat and humidity interact to push animals beyond their physiological limits,” said Cunningham. “This has profound implications for predicting which species and ecosystems are most at risk, and for planning effective conservation responses.”
Researchers studied blue waxbills (Uraeginthus angolensis), small songbirds that were hardest hit in South Africa’s first recorded heat-related bird mortality event in late 2020. By testing their ability to cope with heat in dry and humid conditions, the team found that humidity reduced the birds’ capacity to cool themselves through evaporation. In dry air, waxbills could survive temperatures up to nearly 48 °C, but in humid air their limit dropped to 45.7 °C — the same conditions during a deadly 2020 KwaZulu-Natal heatwave that killed dozens of birds and bats.
Lead author Nazley Liddle, a master’s student at UCT’s FitzPatrick Institute DSI-NRF Centre of Excellence, said the study showed that humidity was a key but often overlooked factor in predicting climate-related bird deaths.
“By the end of the century, the risk of fatal overheating for waxbills could be three to seven times higher in some parts of southern Africa once humidity is factored in,” said Liddle.
The findings suggest that many small birds in hot, humid environments worldwide may be more vulnerable to mass die-offs than previously thought. This challenges existing conservation models, many of which focus only on temperature and overlook the combined stress of heat and humidity.
Senior author Prof Andrew McKechnie, from UP, concurred.
“Humidity limits evaporative cooling, leaving birds with dangerously narrow safety margins,” said McKechnie.
“As climate change intensifies, more areas could become uninhabitable for small songbirds, including the eastern lowlands of southern Africa.”
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