Dec 4, 2022

Mawazo Writing Africa

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Hotter Kalahari Desert may stop hornbills breeding by 2027

Rapid climate change has the potential to affect animal physiology, behavior and breeding success. Research shows, for example, that rising temperatures have negative effects on animals. These range from mass die-offs during heat waves to less obvious problems such as difficulties in foraging.

For arid-zone birds, rising temperatures pose a significant problem. Birds in these arid zones typically breed in response to rain, which often occurs during the hottest time of the year. And birds are mostly active during the day when they are exposed to the sun’s heat. This is when their vital reproductive processes take place, such as eg, territorial defence, courtship, foraging for their young, and nest care.

Research suggests that high temperatures over a few days or weeks can have negative effects on foraging and body mass. On the scale of one to two breeding seasons, these effects have a negative impact on breeding performance. This can be reduced by reducing the condition of the offspring or the likelihood that young birds will reach adulthood and reproduce.

The longer-term effects of responses to high temperatures – over decades – are less well understood.

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Our recent research aimed to fill this knowledge gap. We studied the effects of air temperature and drought on breeding performance of southern yellow-billed hornbills (Tockus leucomelas) in the Kalahari Desert of southern Africa over a decade from 2008 to 2019.

We found breeding performance from our study population dropped during of the monitoring period and correlated strongly with temperature and precipitation. In the Kalahari, air temperatures have already risen by more than 2°C in just a few decades. At this rate, these birds will no longer breed in this location at all by 2027.

Desert Temperatures and Breeding Birds

First, we examined air temperature and precipitation data from the SA Weather Service for the Kalahari region between 1960 and 2020. The frequency and severity of droughts have not changed, but spring and summer average daily maximum air temperatures have increased. Since the mid-1990s they have risen from around 34 °C to well over 36 °C. 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 the breeding performance of a population of southern yellow-billed hornbills in the Kalahari from 2008 to 2019. These birds are still distributed across much of central and eastern southern Africa.

The study population consisted of approximately 25 pairs each breeding season. These hornbills nest in burrows, and at our study site, pairs typically attempted to breed once per season. Their breeding strategy is unusual: the female seals herself in the nest cavity and molts all her flight feathers. This leaves the male parent the sole caretaker of the nest for the female parent and chicks. A successful breeding attempt typically lasts about two months, with a pair raising between one and four chicks.

Although the number of pairs at the site remained constant throughout the study decade, more pairs were skipping the brood each year. And those that bred did so less and less successfully, producing fewer offspring or no offspring at all – the average percentage of nest boxes occupied dropped from 52% to 12%. Nest success – a breeding attempt in which at least one chick was successfully reared – fell from 58% to 17%. The young birds produced per breeding attempt decreased from 1.1 to 0.4.

Without successful breeding, the population cannot survive and will quickly become extinct locally.

We found that the breeding performance was negatively correlated with rising air temperatures and the occurrence of drought during the breeding season. Breeding attempts all failed when average daily maximum air temperatures exceeded 35.7 °C. And the effects of high air temperatures were present even in years without drought.

Considering the strong negative correlation between high air temperature and breeding performance, we argue that global warming is likely the main reason for the recent rapid drop in breeding success in our study population. The effects of high air temperatures (regardless of high rainfall) and drought on the parents affect the likelihood that the offspring will successfully fledge or even attempt to breed.

Based on current warming trends, the 35.7° is C-threshold for successful breeding attempts will be exceeded for the entire hornbill breeding season through about 2027 at our study site.

Overall, although our study is specific to southern yellow-billed hornbills, we suggest our results are likely to be applicable a number of ways. Even for species that are unlikely to die off in large numbers due to heat, climate change can lead to rapid decline and possibly local extinctions.

What we can do about it

Fortunately there are some containment strategies still available to prevent local and global extinctions.

In the short term there are options such as providing water and isolated nesting boxes.

Long term it would be necessary to Conserving habitats that are warming less rapidly or that can mitigate the impacts of climate change on biodiversity.

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 knowledge of how birds deal with heat, suggest that rare and endangered species will be lost over the next century. But also species that are widespread today, such as the southern yellow-billed hornbill.

— Pattinson is a PhD student at the FitzPatrick Institute of African Ornithology, University of Cape Town

  • This article was first published by The Conversation