A southern yellow-billed hornbill in South Africa, October 25, 2017. Photo: Bob Adams/Wikimedia Commons, CC BY-SA 2.0
- For dryland birds, rising temperatures can affect life processes – territorial defence, courtship, finding food and caring for the nest – for breeding to take place.
- A study of southern yellow-billed hornbills showed that breeding production was negatively correlated with increasing air temperatures and the occurrence of drought during the breeding season.
- These results will apply to other species. Even for species that are unlikely to die in large numbers from heat, climate change can cause rapid declines.
Rapid climate change has the potential to strongly influence the physiology, behavior and reproductive success of animals. Research shows that rising temperatures, for example, have negative effects on animals. These range from mass mortality events during heat waves to less obvious issues like difficulty finding food.
For dryland birds, rising temperatures pose a significant problem. Birds in these dry areas typically breed in response to rainfall, which often occurs during the hottest time of the year. And the birds are most active during the day, when exposed to the heat of the sun. This is when their vital reproductive processes take place – such as defending territory, courtship, finding food for their young, and frequenting the nest.
Research suggests that high temperatures for a few days or weeks can have negative effects on foraging and body mass. At the scale of one or two breeding seasons, these effects have a negative impact on reproductive performance. This can be done by reducing the condition of offspring or the likelihood of young birds surviving to adulthood and reproducing.
The longer-term effects of high temperature responses – over decades – are less well known.
Our recent research aimed to help fill this knowledge gap. We assessed the effects of air temperature and drought on the reproductive output of southern yellow-billed hornbills (Tockus leucomelas) in the Kalahari Desert in Southern Africa over a ten-year period from 2008 to 2019.
We found that the reproductive output of our study population plummeted during the monitoring period and was strongly correlated with temperature and precipitation. In the Kalahari, air temperatures have already increased by more than 2°C in a few decades. At this rate, by 2027, these birds will not breed at this site at all.
Desert Temperatures and Breeding Birds
We first looked at air temperature and precipitation data from the South African Meteorological Service for the Kalahari region between 1960 and 2020. Drought frequency and severity have not changed, but temperatures Mean daily air maxima in spring and summer have increased. They have gone from around 34°C to well over 36°C from the mid-1990s to the present day. [This is from our paper] This equates to a warming rate of about 1°C per decade, a rate five times faster than the global average of about 0.2°C per decade.
We then assessed the effects of air temperature and drought on the breeding output of a population of southern yellow-billed hornbills in the Kalahari during the decade 2008 to 2019. These birds are still common in much of central and eastern southern Africa.
The population studied consisted of about 25 pairs in each breeding season. These hornbills nest in cavities and at our study site, pairs generally make one breeding attempt per season. Their reproductive strategy is unusual: the female seals herself inside the nest cavity and moults all her flight feathers. This leaves the male parent as the sole provider of the nest for the female parent and the chicks. A successful breeding attempt usually takes about two months, with a pair raising between one and four chicks.
Although the number of pairs at the site remained constant over the decade of study, more pairs skipped breeding each year. And those that reproduced did so less and less well, producing fewer offspring or none at all.
Comparing the first three monitoring seasons (2008-2011) to the last three (2016-2019), the average percentage of nest boxes occupied fell from 52% to 12%. Nest success – a breeding attempt that successfully raises at least one chick – fell from 58% to 17%. The number of chicks produced per breeding attempt decreased from 1.1 to 0.4.
Without successful reproduction, the population will not be able to persist and will rapidly die out locally.
We found that breeding production was negatively correlated with increasing air temperatures and the occurrence of drought during the breeding season. Breeding attempts all failed when mean daily maximum air temperatures exceeded 35.7°C. And the effects of high air temperatures were present even in years without drought.
Given the strong negative correlation between high air temperature and reproductive output, we argue that global warming was likely the main driver behind the recent and rapid collapse in reproductive success in our study population. . The consequences of high air temperatures (regardless of heavy rainfall) and drought on parents affect the likelihood of successfully fledging offspring or even attempting to reproduce.
Based on current warming trends, the 35.7°C threshold for successful breeding attempts will be exceeded throughout the hornbill breeding season of approximately 2027 at our study site.
Overall, although our study is specific to southern yellow-billed hornbills, we suggest that our results are likely applicable to a range of species. Even for species that are unlikely to die in large numbers from heat, climate change can lead to rapid declines and potentially local extinctions.
What we can do about it
Fortunately, a few mitigation strategies are still available to help prevent local and global extinctions.
In the short term, there are options such as providing water and insulated nest boxes.
In the long term, habitats that warm up less quickly or that can cushion the effects of climate change on biodiversity should be preserved.
However, even habitat preservation will not be enough if the current rate of climate change continues. Recent models based on current rates of warming and what is known about how birds respond to heat suggest that rare and endangered species will be lost over the next century. But so will currently common species, such as the southern yellow-billed hornbill.
Nicholas B. PattinsonPhD student FitzPatrick Institute of African Ornithology, University of Cape Town.
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