By Amanda Bourne
During my doctoral research, I explored the impacts of high temperatures and drought on a desert-adapted bird – the Southern Pied Babbler Turdoides bicolor – in the Kalahari Desert of southern Africa. High temperatures and drought, both increasing with climate change, inhibit successful breeding in pied babblers and the interannual survival of two important age classes: juveniles recruiting into the adult population and experienced breeding adults.
Anthropogenic climate change has altered weather patterns in every ecosystem on Earth. Average temperatures are increasing at an unprecedented rate and are projected to continue to increase over most land masses in the future. Changes to the timing and predictability of rainfall and the frequency and intensity of extreme rainfall events are also being observed. The resulting increase in harsh and unpredictable climate regimes has far-reaching consequences for birds around the world. Impacts directly attributable to climate change include higher risk of mortality, reduced breeding success, declining populations, and potentially maladaptive changes to foraging, parental care, and migration.
The incidence of both high temperature extremes and drought in the Kalahari is expected to increase in severity and frequency as climate change advances. For my research on pied babblers I used behaviour, morphology, and physiology data that I collected over three consecutive austral summer field seasons (2016-2019) along with Assoc. Prof. Amanda Ridley’s ~15-year life history dataset (2003-2019, to which I contributed the last three years of data). I investigated the impacts of temperature and rainfall on this species’ interannual survival, behaviour, and reproduction, taking a multidisciplinary approach combining behavioural ecology, life history study, and ecophysiology.
Pied babblers are medium-sized (60–90 g), sexually monomorphic, cooperatively-breeding passerines endemic to the Kalahari in southern Africa. They are highly social year-round residents, living in territorial groups ranging in size from 3-15 adults (individuals > 1 year old). Groups consist of a single breeding pair with subordinate helpers that are often, but not always, the offspring of the pair. All adult group members participate in cooperative behaviours, including territory defence, sentinel duties, and parental care. Dominant pairs monopolise ~95% of breeding activity and can be identified through their breeding behaviour. For example, only dominant females incubate the nest overnight and the dominant pair have a distinctive duetting call.
Pied babblers breed during the austral summer, typically between August and April, when it is hottest. They nest in camelthorn Vachellia erioloba or blackthorn Senegalia mellifera trees, usually at heights of 3-10 m. Pied babbler groups typically attend to one nest at a time, but they may attempt to breed several times within a single breeding season and the dominant pair may initiate and incubate a new clutch while the group is still feeding dependent fledglings from the previous breeding attempt. Previous research on this species has shown that high temperatures and drought affect population demographics, increasing the risk of localised extinction, reducing offspring provisioning rates, and constraining foraging behaviour. High temperatures are also associated with an inability to maintain body mass between consecutive days and reduced investment in territorial defence in this species.
Although acute, lethal effects of high temperatures and drought as a direct result of dehydration, starvation, and hyperthermia do occur in wild populations, they are relatively uncommon. Far more common, and with potentially pernicious long-term consequences that are not yet fully understood, are the chronic, sub-lethal effects of high temperatures and drought acting on individuals via effects on body condition, reproduction, foraging efficiency, interannual survival and, ultimately, population persistence. In my research I found strong evidence for chronic and severe sub-lethal effects of high temperatures and drought on reproduction and interannual survival in pied babblers.
The total number of days during the breeding season (Oct–Mar) that exceed 35.5 °C, identified previously as a critical temperature threshold for mass maintenance and parental investment in pied babblers, has been increasing at my Kalahari field site, and rainfall has been decreasing. High mean air temperatures (>35.5 °C during incubation, >37.3 °C for nestlings, and >36.6 °C for dependent fledglings) were significantly associated with higher mortality risk during early development in pied babblers. I identified a thermal limit of 38 °C (mean daily maximum temperature), above which we recorded no successful breeding across 15 years of data for the study population. High temperatures between fledging and nutritional independence were associated with reduced body mass gain and lower recruitment of juveniles into the adult population. High temperatures during the breeding season were also associated with body mass loss and reduced interannual survival in breeding adults. In summary, high temperatures were a dominant driver of reproductive failure and mortality during all life stages from egg to breeding adult.
I also investigated the impacts of other climate factors, and how these interacted with high temperatures to influence survival and reproduction in pied babblers. Higher rainfall positively influenced nest outcomes and the number of surviving young per breeding attempt. This result suggests that the greater availability of food resources following rain could at least partially buffer the impacts of high temperatures and are consistent with other studies in which breeding birds respond flexibly and opportunistically to changes in their environment.
Reproductive effort and success were both lower during breeding seasons characterised by drought. When droughts (breeding seasons with ≤135.75 mm of rainfall between September and March) occurred in combination with high average daily maximum temperatures (> ~38 °C), the effect of temperature was exacerbated, and interannual survival of both juveniles and breeding adults was severely compromised. Specifically, simultaneously hot and dry conditions were associated with an 86% reduction in interannual survival amongst juvenile pied babblers and a 60% reduction in interannual survival amongst breeding adults.
With temperatures rising fast in the Kalahari, and hot droughts predicted to become ever more frequent and severe, pied babblers may increasingly experience conditions that inhibit both successful breeding and the interannual survival of two important age classes: juveniles recruiting into the adult population and experienced breeding adults. The impacts of hot droughts in the interannual survival of experienced breeding adults is particularly concerning as it is these birds that have the greatest potential to impact on population growth rates during good years, influencing recovery after extreme events. Pied babblers are relatively long-lived and well adapted to the natural variability of their arid environment, but an increase in the frequency and severity of hot weather extremes, especially hot droughts, could undermine population recovery after extreme events, and population growth and persistence overall, leading ultimately to localised extinctions for this species.
I found some evidence that pied babblers compensated for periodic droughts by increasing their investment in breeding in years following droughts. This is likely to be an important part of both long-term population persistence and post-drought population recovery. However, given that rapid increases in the frequency and severity of droughts are predicted under climate change, compensatory mechanisms which have been successful at maintaining populations in the past may prove insufficient to allow population recovery between extreme events in the future. What can we do to limit the impacts of climate change on birds such the pied babbler? The improved understanding of the ways in which birds like pied babblers respond to high temperatures and drought now that we now have will inform population viability models and targeted conservation management actions, which is good. But what is required is a large and urgent reduction in carbon emissions globally, and the conscientious preservation of the habitat on which these birds and others like them depend.
Written by Amanda Bourne, PhD Candidate at the Fitzpatrick Institute of African Ornithology, University of Cape Town, South Africa. Amanda’s PhD was supervised by Dr Susan Cunningham (Hot Birds Research Project, UCT), Assoc. Prof. Amanda Ridley (Pied Babbler Research Project, UWA), and Prof. Claire Spottiswoode (African Honeyguide Research, Cambridge University).
Bourne AR, Cunningham SJ, Spottiswoode C, Ridley AR. 2020. High temperatures drive offspring mortality in a cooperatively breeding bird. Proceedings of the Royal Society B 287: 20201140
Bourne AR, Cunningham SJ, Spottiswoode C, Ridley AR. 2020. Hot droughts compromise interannual survival across all group sizes in a cooperatively breeding bird. In press, Ecology Letters
Bourne AR, Cunningham SJ, Spottiswoode C, Ridley AR. 2020. Compensatory breeding in years following drought in a desert-dwelling cooperative breeder. Frontiers in Ecology and Evolution 8: 190