Green-eared barbet. (Photo: Flickr)
Heat-associated mortality events are common in areas like the Australian Outback, but a recent incident in South Africa that killed nearly 50 birds in a day has scientists worried.
It was in the searing heat of the afternoon when the staff at the Pongola Nature Reserve noticed that something was wrong.
Throughout the morning of 8 November 2020 in this corner of northern KwaZulu-Natal the temperature just kept rising.
By 10am the thermometer at the administration office registered 40°C. In mid-afternoon the temperature settled at 45°C and it was then that the staff in the nature reserve noticed dead and dying birds around their office buildings.
The following day rangers collected the corpses of 47 birds during an 11km patrol through the nature reserve.
What had happened the day before, scientists quickly realised, was South Africa’s first known heat-related die-off.
It didn’t just claim birds, it killed bats too.
These heat-associated mortality events usually happen in the scorching Australian Outback, and their arrival in South Africa has Professor Andrew McKechnie worried.
“Yes, I believe it is climate change and the reality is that climate change is increasing the frequency and the intensity of heatwaves,” the professor at the University of Pretoria’s Department of Zoology and Entomology says.
“And this is part of the same pattern we’ve just seen in Canada within the last week. It’s mind blowing. If someone had told me 20 years ago that in the year 2021 a town east of Vancouver would experience almost 50 degrees Celsius, I wouldn’t have believed it.”
Over the last 11 years McKechnie has been involved in the Hot Birds Research Project, an international collaboration of scientists studying the effects of heat and climate change on desert bird species. The project spans the arid southern US, the Australian Outback and the Kalahari.
In that time scientists have noticed behavioural changes in the species they have studied.
“We have evidence now across a number of species, both in the Kalahari and in other arid zone ecosystems in Australia; in the US as well,” says Dr Susan Cunningham, a senior lecturer at the FitzPatrick Institute of African Ornithology and a member of the Hot Birds Research Project.
“This evidence shows that some bird species are spending more time trying to stay cool as they deal with increased numbers of hot days. Birds are forced to shelter in the shade when they should be foraging.”
In the Kalahari, one bird species that appears to have been affected by rising temperatures is the southern pied babbler.
“We have got over 15 years of data on these birds that has allowed us to track changes through time, and we can see that these hot summers are causing problems for those birds in terms of their ability to get enough food into their chicks and their ability to survive the following winter after a hot summer,” explains Cunningham.
What is driving this, believe the scientists, is a dramatic increase in the number of days in the Kalahari with temperatures above 35°C. Weather stations in the southern Kalahari have shown that this trend began in the mid-1990s.
“The number of days of heat above 35 degrees used to sit around 90 days per year, and since the mid-1990s that’s gone up to above 120 days. And we are looking at about 130 now,” Cunningham says.
For McKechnie the fear is that an event like the one in Pongola last year could be catastrophic for certain bird and mammal species.
“If you had a heatwave on the same sort of scale [as Pongola] that affected some range-restricted species, for example the green barbet, you could potentially lose a large part of the population of that entire species within the day.”
There have been species-threatening heatwave mortality events before. In November 2018 a two-day heatwave in Cairns, Australia, wiped out a third of Australia’s entire population of spectacled flying foxes.
But what was responsible for the killings in both the Pongola and Cairns die-offs wasn’t the heat, it was the humidity.
“Humidity complicates things because birds are relying on evaporating water to keep cool,” says McKechnie.
What is not known is how many birds died in Pongola. The rangers, it was later estimated, covered only about 1% of the reserve on the patrol following the heat event.
The focus of the Hot Birds Research Project is shifting. McKechnie and his colleagues are beginning to move their study away from deserts and are examining the effects of humidity on birds. Their research also looks at interventions that help threatened species survive heatwaves. These could be as simple as providing sheltered nest boxes or water baths.
But for the moment McKechnie is keeping an eye on that part of the world that has just experienced the latest extreme heatwave – Canada. It was reported that large numbers of oysters and mussels died from the heat along the British Columbia coastline, but he hadn’t yet heard of bird deaths.
“But with something like this, there had to have been bird die-offs,” he says. DM/OBP
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