It was an extremely hot day on 8 November last year in northern KwaZulu-Natal — so hot that it killed at least 110 birds and fruit bats. Staff at the Phongolo Nature Reserve found large numbers of dead and dying birds, mostly songbirds, around the reserve’s headquarters, while Wahlberg’s epauletted fruit bats were found dead in Pongola itself and Hluhluwe.
On 1 December, the Mail & Guardian reported how dozens of birds and bats perished on that scorching day when temperatures reached an unusual 40°C by 10am and climbed to a searing 45°C.
“That a significant heat-related mortality event has now been documented in southern Africa is cause for concern, supporting recent predictions that climate change is leading to a substantially greater risk of lethal effects of extreme weather events among small endotherms [warm-blooded animals] globally,” he writes in a new paper on the event, with colleagues from Ezemvelo KZN Wildlife and the FitzPatrick Institute of African Ornithology.
As far as the team is aware, this is the first documented heat-related mortality event involving wild birds and bats in southern Africa.
On 8 November, air temperatures reached 43°C to 45°C and relative humidities were 21% to 23%, the authors write in their paper, published in the journal Austral Ecology.
“This mortality event occurred on a single very hot day preceded by several cooler days (37°C to 39°C at one location) and involved weather conditions similar to those associated with at least one recent flying-fox die-off in Australia.”
The FitzPatrick Institute of African Ornithology says that South Africa “passed a grim milestone” on 8 November in terms of the effects of global heating on birds.
“For the first time the country recorded a mass mortality event of birds and bats linked to an extreme heatwave in northern KwaZulu-Natal …. Devastating effects like this are likely to become more common as climate warming progresses.”
The researchers describe how the recorded deaths are likely only a small fraction of the total number of individuals involved, as only carcasses close to human habitation and within the roughly 45ha search area in the reserve were recorded.
The deaths included 47 birds of 14 species, mostly songbirds, and about 60 Wahlberg’s epauletted fruit bats.
“Heat dissipation behaviour (panting, seeking of shade on the south side of buildings, drinking or bathing) and subsequently dead and dying birds were first noted in the vicinity of the Phongolo Nature Reserve office complex.”
The following day, field rangers walked a distance of 11km searching for further casualties, with most victims found in the reserve.
Small numbers of fruit bat deaths were also reported from Hluhluwe village and at a lodge within Manyoni Private Game Reserve. In addition, the carcasses of six small insectivorous bats were found in a hangar at Hluhluwe airstrip.
“Multiple reports were received of atypical avian drinking behaviour on the same day. At Zebra Hills Lodge within the Manyoni Private Game Reserve, for instance, unprecedented numbers of birds were observed drinking and bathing at a small pool of water in a shaded location.
“The species involved included several small insectivores that do not usually drink, such as the grey tit-flycatcher and willow warbler.”
While this was the first such event recorded for southern Africa, extreme heatwave events like this are already common in Australia, and are likely to become more common in southern Africa in the future, the researchers warn.
“Extreme heat events sometimes result in large-scale mortality among animals. Avian mortality events are best known in Australia, and in at least one instance had serious negative consequences for a threatened species,” according to the paper.
“Among Pteropodidae bats, catastrophic die-offs involving tens of thousands of flying foxes have become regular occurrences on the Australian east coast, sometimes decimating populations.”
The researchers describe how, when environmental temperature exceeds body temperature, evaporative cooling becomes the only avenue available for heat loss.
“Mortality can occur if animals are unable to evaporatively dissipate heat fast enough to defend body temperature below lethal limits, or if cumulative evaporative water losses exceed dehydration tolerance limits.”
The frequency of hyperthermia or dehydration-associated avian mortality events is projected to increase substantially in arid regions during the 21st century. “Increasingly frequent mortality events are anticipated among flying foxes, for which air temperature exceeding 42°C has emerged as a strong predictor of mortality.”
As no carcasses were collected for post-mortem investigations, the authors say they cannot completely rule out potential causes of mortality such as disease, poisoning or contaminated drinking water.
But several factors point to a direct link between weather conditions and the observed mortality.
Among these are that the deaths at multiple locations up to 90km apart on the same day make it “highly unlikely” that poisoning or contaminated water sources were involved, as do the accounts of atypical drinking behaviour and visible signs of heat stress observed among birds at the two nature reserves.