An iconic Southern African bird — the Southern yellow-billed hornbill — could be wiped out in the hottest parts of its range in the arid Kalahari Desert by 2027 because of rapid warming from climate change.
This is the grim conclusion of a decade-long study by the University of Cape Town (UCT), which investigated the effect of rapid climate change on the breeding success of a population of yellow-billed hornbills at the Kuruman River Reserve in the Northern Cape.
Over the monitoring period from 2008 to 2019, the scientists found that the breeding success collapsed in line with rapid warming in the region. They first examined air temperature and rainfall data for the Kalahari region between 1960 and 2020.
“While the return rate and severity of drought has not changed, air temperatures have been increasing at approximately 1°C per decade since the mid 90s — an alarmingly rapid rate,” said Nicholas Pattinson, a PhD candidate at UCT’s Fitzpatrick Institute of African Ornithology.
The scientists then assessed the effects of air temperature and drought on the yellow-billed hornbills breeding in nest boxes.
“We found that the breeding output of our study population collapsed during the monitoring period. Comparing the first three seasons (2008-11) of monitoring to the last three (2016-19), the mean percentage of nest boxes occupied declined from 52% to 12%, nest success from 58% to 17%, and mean fledglings produced per breeding attempt from 1.1 to 0.4,” Pattinson said.
The primary finding concerning the collapse in breeding was that breeding output was negatively correlated with increasing air temperatures and the occurrence of drought within the breeding season, he said: “The effects of high air temperatures were present even in non-drought years; of the 115 breeding attempts we recorded, all 18 attempts during which average daily maximum air temperatures exceeded 35.7°C, failed.
“Considering the strong negative correlation between high air temperature and breeding output, we argue that global warming has likely been the primary driver of the recent, rapid collapse in breeding success in our study population. Essentially, the sub-lethal consequences of high air temperatures (regardless of high rainfall) and drought on the parents affect the probability of successfully fledging offspring or even attempting to breed at all.”
Based on current warming trends the average daily maximum air temperature threshold of 35.7°C, above which no successful breeding attempts were recorded, will be exceeded over the course of the entire hornbill breeding season by about 2027 at the study site, Pattinson said.
Current climate change predictions make it “very unlikely” that hornbills will persist across the hottest parts of their range, even over the next decade. “However, if they are going to occur anywhere across their current distribution in the future, the temperatures will have to remain below this threshold of 35.7°C during their breeding.”
While the study is specific to Southern yellow-billed hornbills, the findings are probably applicable to a range of species, he said. “Our findings support the proposition that even for species for which catastrophic heat-related mass die-off events remain unlikely, climate change can drive rapid declines and potentially local extinctions.”
For arid zone birds, in particular, increasing temperatures pose a significant problem, since birds are commonly constrained to breed in response to rainfall, which often occurs during the hottest time of the year. “In addition, birds are mostly active above ground during the day, so their vital processes for reproduction such as territorial defence, courtship, provisioning and attending the nest must take place through the heat of the day, risking exposure to high heat loads in the sun.”
The short-term effects of birds’ responses to high temperatures and heat stress are becoming increasingly apparent. “Research suggests that high temperatures over a period of a few days to weeks can have negative effects such as reducing foraging success, altering predation pressure and impairing body mass maintenance among others.”
At the scale of one or two breeding seasons, these effects have been noted to negatively affect breeding performance, either through reducing offspring condition or the probability of offspring survival and recruitment into the population as breeding adults.
Fortunately, he said, several mitigation strategies are still available to help prevent local and global extinctions. “In the short term, some options such as providing water sources and enhanced nest sites (for example, nest boxes designed to keep the nest cool and well insulated) might be useful stop-gap options while we find solutions to the bigger issues.”
Long term, habitat preservation is an excellent goal. “Specifically, habitat which is projected to provide a ‘climate refuge’ (areas warming less rapidly, or pristine habitat, which may buffer the effects of climate change on the biodiversity present) over the coming century might be highly prioritised.”
But even habitat preservation will only go so far if the current rate of climate change is sustained. “Recent models based on current rates of warming and what we know about how birds handle heat are suggesting that we will not only lose rare and endangered species over the next century, but species which are currently common such as the Southern yellow-billed hornbill. In the end, a global effort to curb the climate crisis is absolutely, unavoidably critical, not just for the future, but for what is happening under our noses right now.”