Field rangers in the Phongolo Nature Reserve in South Africa started discovering dead birds on the ground in late 2020. The temperature had risen above 45 degrees Celsius, which is about ten degrees higher than the worst temperatures the area typically experiences. Small species like the blue waxbill, a delicate finch with a powder-blue face that forages in grass in small, agitated flocks, were among the birds most impacted. Nearly half of the carcasses found by rangers searching a portion of the reserve after the heat broke were blue waxbills.
It would be simple to interpret that as a singular incident, a bad week in one region of one nation. However, scientists at the University of Pretoria and the University of Cape Town, who had been researching the effects of climate change on African birds since 2009, had a different perspective. For them, Phongolo was a piece of evidence that supported a long-standing concern: the combination of heat and humidity creates a physiological trap from which small birds are unable to flee.
The consequences are complex, but the science underlying it is not. Like most animals, birds use evaporative cooling to release body heat when temperatures rise. They wear pants. They reveal skin. They look for shade. The issue is that evaporation is only effective in situations where the surrounding air is sufficiently dry to absorb moisture. The body’s cooling system essentially stalls in humid conditions because the air is already nearly saturated. In dry heat, a blue waxbill can withstand air temperatures as high as 48 degrees Celsius. That threshold falls to about 45.7 degrees in humid conditions like those experienced during the Phongolo event. Hundreds of birds were killed by that difference, which was only two and a half degrees.
Subsequent modelling by the same research team found that when humidity is factored in alongside rising temperatures, predicted mortality risk for birds like the blue waxbill becomes three to seven times higher than temperature projections alone would suggest. It’s possible that many assessments of future bird mortality have been underestimating the danger by working from temperature data without adequately accounting for the role of moisture in the air.
Africa is not the only continent where this issue exists. More than 90,000 observations from more than 3,000 bird populations worldwide were examined in a comprehensive study that was published in Nature Ecology and Evolution in 2025.
The study compared population trends with daily weather records dating back to 1940. The results were difficult to ignore: since 1950, tropical bird populations have decreased by 25 to 38 percent due to exposure to extreme heat. Between the late 1970s and 2020, populations of the majority of species in two undisturbed rainforests—one in Panama and one in the Amazon—dropped by more than 50%. These were not forests touched by logging or farming. By most accounts, they were unharmed.
There’s a feeling, reading through this body of research, that the scientific community has been watching a slow emergency build for decades while the broader conversation focused elsewhere. Building collisions, pesticides, and habitat loss are all real and dangerous.
But extreme heat events, multiplied in frequency and intensity by fossil fuel emissions, appear to be driving declines even where other pressures are absent. Tropical birds have been experiencing dangerously hot days roughly ten times more often than they did sixty years ago. Trumpeter hornbills, large fruit-eating forest birds whose seed dispersal quietly holds rainforest ecosystems together, are facing sharply elevated mortality risk across parts of southern and central Africa in the coming decades.
What’s left, then, is the uncomfortable arithmetic. The only way to truly address the underlying cause is to reduce greenhouse gas emissions. If the atmosphere continues to warm and the wet-season air in tropical forests continues to become more difficult for small birds to breathe in, conservation efforts, habitat preservation, and captive breeding programs will not address the fundamental issue.
