Rapid global warming is driving southern yellow-billed hornbills to local extinction

In a study published in the journal Frontiers in Ecology and Evolutiona team of scientists from South Africa assessed the effects of air temperature and drought on the reproductive output of southern yellow-billed hornbills (Tockus leucomelas) in the Kalahari Desert.

The southern yellow-billed hornbill (Tockus leucomelas), adult female, in Mapungubwe National Park, South Africa. Image credit: Derek Keats / CC BY 2.0.

Global warming is exacerbating the harsh conditions associated with arid environments by raising average air temperatures and increasing the frequency and intensity of heat waves and drought.

Possible consequences for animals living in arid regions include an increased frequency of mass mortality events and disastrous reproductive failures.

However, heat waves, especially in association with droughts, can also have insidious sublethal effects, including loss of body condition, reduced egg or clutch size in birds, reduced foraging rates and compromised offspring quality, and total abandonment of reproduction.

“There is growing evidence of the negative effects of high temperatures on the behavior, physiology, reproduction and survival of various species of birds, mammals and reptiles around the world,” said Dr Nicholas Pattinsonresearcher at the University of Cape Town.

“For example, heat-related mass mortality events over a period of days are increasingly being recorded, which undoubtedly poses a threat to population persistence and ecosystem function.”

The southern yellow-billed hornbill’s distribution includes most of southern Africa, much of it being in the Kalahari Desert.

Known for its particular breeding and nesting strategy, this bird is a socially monogamous species.

Southern yellow-billed hornbills nest in cavities; the female seals herself in the nest cavity and stays there for an average of 50 days to brood and care for the chicks. The only opening is a narrow vertical slit, through which the male feeds the female and the chicks.

This type of nesting largely protects against predation, which means that breeding success mainly depends on other factors such as climate and food availability.

For example, yellow-billed hornbills initiate breeding in response to rainfall, which corresponds to the hottest days of the year. It is therefore difficult for them to move breeding dates outside of the hottest periods.

Dr Pattinson and his colleagues studied a population of southern yellow-billed hornbills in the Kuruman River Reserve in South Africa’s southern Kalahari Desert between 2008 and 2019.

They collected data from pairs breeding in wooden nest boxes. They examined breeding success at both broad and fine scales and analyzed climatic trends in the region.

The results showed that livestock production plummeted during the monitoring period due to the increase in maximum air temperature.

“During the monitoring period, the sublethal effects of high temperatures (including compromised foraging, provisioning and maintenance of body mass) reduced the chances of hornbills breeding successfully or even breeding at all,” Dr. Pattinson said.

By comparing the first three seasons (between 2008 and 2011) to the last three (between 2016 and 2019), the researchers found that the average percentage of occupied nest boxes fell from 52% to 12%, nest success (rearing and fledging less than one chick) decreased from 58% to 17%, and the average number of chicks produced per breeding attempt decreased from 1.1 to 0.4.

No successful breeding attempts have been recorded above the air temperature threshold of 35.7 degrees Celsius.

Breeding output was negatively correlated with increasing the number of days that maximum air temperature exceeded the threshold at which hornbills displayed heat dissipation behavior and normal breeding and nesting behavior. These effects were present even in drought-free years.

The study shows that the rapid pace at which the climate crisis is unfolding is having severe negative effects on charismatic species over alarming time periods

Current warming predictions at the study site show that the hornbill’s threshold for successful breeding will be exceeded throughout the breeding season of approximately 2027.

“Much of the public perception of the effects of the climate crisis is tied to calculated scenarios for 2050 and beyond,” Dr Pattinson said.

“Yet the effects of the climate crisis are present and can manifest not just within our lifetime, but even over a single decade.”

“Despite the absence of large mass mortality events, our prediction in this study is that southern yellow-billed hornbills could disappear from the warmer parts of their range as early as 2027.”

“The sublethal consequences of high temperatures can lead to local extinctions by leading to recruitment failure (i.e. no young animals joining the population) and changes to the ecosystems we all depend on.”

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Nicholas B. Pattinson et al. Collapse of breeding success in desert hornbills evident in a single decade. Before. School. Evol, published online May 19, 2022; doi: 10.3389/fevo.2022.842264

Teresa H. Sadler