Researchers studied blue waxbills — small songbirds that were most affected in South Africa’s first documented heat-related mortality event involving wild birds, which occurred in late 2020. (Wikimedia Commons)
High humidity significantly worsens the threat of lethal hyperthermia in small birds during extreme heat events, new research has found.
The collaborative study by the University of Cape Town (UCT), the University of Pretoria, the University of the Witwatersrand and the South African National Biodiversity Institute (Sanbi) raises serious concerns about the survival of species in hot, humid regions under climate change.
It also underscores the importance of the country’s research institutions working together to understand and mitigate 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 associate professor Susan Cunningham, the director of UCT’s FitzPatrick Institute of African Ornithology.
The researchers studied blue waxbills — small songbirds that were most affected in South Africa’s first documented heat-related mortality event involving wild birds, which occurred in late 2020.
On 8 November that year, about 50 birds and 60 bats were found dead around Pongola in northern KwaZulu-Natal after temperatures reached 45°C.
By measuring the waxbills’ ability to tolerate heat under both dry and humid conditions, the team found that humidity significantly reduced their capacity to cool themselves through evaporation.
In dry air, waxbills could withstand air temperatures of nearly 48°C. But in humid air, their heat tolerance limit dropped by more than 2°C, to just 45.7°C — the same conditions recorded during the KwaZulu-Natal heatwave.
“Humidity is a critical, but often overlooked, factor in predicting climate-driven bird mortality,” said lead author Nazley Liddle, a master’s student at the FitzPatrick Institute’s department of science and innovation-National Research Foundation Centre of Excellence.
“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.”
The results suggest that many small birds in hot, humid environments worldwide may be more vulnerable to mass die-offs than previously thought — challenging conservation models that focus only on temperature and overlook the combined stress of heat and humidity.
The study reinforces calls for climate adaptation strategies that incorporate humidity into wildlife vulnerability models to support targeted conservation responses.
“Humidity limits evaporative cooling, leaving birds with dangerously narrow safety margins,” said Andrew McKechnie, a professor in the department of zoology and entomology at the University of Pretoria, and the co-principal investigator of the international Hot Birds Research Project.
“As climate change intensifies, more areas could become uninhabitable for small songbirds, including the eastern lowlands of southern Africa.”
15 years of the Hot Birds Research Project
Last week, McKechnie presented findings from 15 years of the Hot Birds Research Project at the Oppenheimer Research Conference, where he spoke to the Mail & Guardian on the sidelines.
Convened by Oppenheimer Generations Research and Conservation, the conference brought together researchers, policymakers and conservation leaders to advance African-led solutions to the biodiversity and climate crisis.
“We started very much focused on two fields: behaviour and physiology. And there have been a lot of insights we’ve gained into how hot weather affects birds,” said McKechnie, who also holds the South African Research Chair in Conservation Physiology at Sanbi.
“Those are broadly classified as direct impacts in terms of birds exceeding physiological tolerance limits; the sort of things that happen during these die-off events where birds just don’t have the physiological capacity to tolerate extreme heat combined with extreme humidity.”
What they have learnt is that species differ tremendously in terms of how well they can handle extreme temperatures. Songbirds, or passerines, which make up about 55% of all bird species on Earth, have relatively modest heat tolerance, he explained.
“In the Pongola event, 94% of the mortalities involved songbirds. We’ve also learnt that humidity is a very big problem for birds in hot, humid environments.”
For years, the project’s research focused on species in hot, arid regions such as the Kalahari Desert, where dry air allows evaporative cooling.
“But when you get into these very humid environments, evaporative cooling just can’t happen and birds lose that last defence against body temperature increasing to lethal limits.”
On the behavioural side, they are seeing more subtle effects of high temperatures that are not as easy to discern, but are nevertheless consequential.
“Many of those impacts have to be with birds trading off heat dissipation behaviours against foraging behaviour. So, for example, on a very hot day, a bird is going to be forced to spend more time resting in the shade …”
High temperatures also drive behavioural changes with long-term impacts. “On a very hot day, a bird is forced to spend more time resting in the shade,” McKechnie said. “That comes at the cost of time spent foraging.
“During very hot weather, we see body mass decreasing by typically 4% or 5% with every successive day.”
In periods of sustained high temperatures around 40°C, it becomes problematic because if a bird is losing 4% of body mass a day, “it’s not going to be many days before it gets into serious trouble. This results in declines in body mass during very hot conditions.”
In yellow-billed hornbills in the Kalahari, fledglings raised during extreme heat sometimes leave the nest weighing half of what they would under normal conditions and are quickly taken by predators.
“These behavioural consequences of heat are a major mechanism linking climate change to population declines,” McKechnie said.
Between 2008 and 2019, breeding in a monitored Kalahari population of yellow-billed hornbills collapsed by about 80% because of heat and drought. Cooler, wetter La Niña years have since driven partial recovery, but “you are seeing these rapid declines, particularly in breeding associated with very hot conditions”, he noted.
Recent studies are documenting sharp declines in tropical bird populations, even in pristine forests. “This is all being driven by rising temperatures,” said McKechnie. “You’re starting to see many of the effects we’ve been predicting for the last decade and a half.”
The Hot Birds Research Project is expanding its work into humid areas. McKechnie said that it’s reasonable to expect that humidity will worsen the impacts of heat, “and that we’ll see even stronger effects on behaviour, foraging rates, body mass and nesting growth in these environments”.