High Humidity Cuts Tropical Bird Heat Tolerance Raising Mass Mortality Risk

Research on blue waxbills and trumpeter hornbills in South Africa shows that high humidity significantly reduces birds' ability to cool themselves through evaporation, lowering the air temperature threshold at which lethal hyperthermia occurs. Climate models predict that mortality risk from combined heat and humidity will be three to seven times higher than from temperature increases alone across parts of Kruger National Park, southern Zimbabwe and Mozambique by mid-century.

In late 2020, temperatures in the humid Phongolo Nature Reserve in KwaZulu-Natal exceeded 45°C and blue waxbills made up nearly half the bird carcasses found after the event. Laboratory experiments on 14,921 individuals from five cavity-nesting bee and wasp species and separate experiments on waxbills across a range of temperature and humidity conditions demonstrated that blue waxbills can tolerate air temperatures up to 48°C in dry conditions but cannot maintain a safe body temperature above 45.7°C under high humidity similar to conditions on the day of the 2020 mortality event.

The mechanism is straightforward: birds cool through evaporation, and when air humidity is already high, sweat and panting cannot transfer heat to the surrounding air efficiently. Core body temperature rises, nervous system function degrades, organ damage begins and proteins denature. The same evaporative cooling failure that makes hot humid days dangerous for humans applies to tropical birds but with a narrower safety margin. Population declines of 25 to 38 per cent since 1950 among tropical birds have been attributed to increasingly extreme heat events, with songbirds showing greater vulnerability than other groups.

The authors from universities including Cape Town and Pretoria project that regions across much of Kruger National Park, south-eastern Zimbabwe and large parts of southern and central Mozambique including the Zambezi Valley will experience sharply elevated mortality risk as warming and humidity combine. Many of these areas will become too hot and humid during wet seasons for affected species to persist. The researchers identify a research gap in understanding how quickly insect and bird populations can genetically adapt to rapidly changing climate conditions, and call for monitoring of wet-bulb temperature as a biodiversity conservation indicator.

Key figure — Mortality risk three to seven times higher when humidity is factored in