There is an upward limit to how hot daily air temperatures can get before birds can no longer successfully breed, indicating that rapidly advancing climate change will have widespread and tragic effects on arid zone bird species
As human-caused climate change continues to cook the planet, its far-reaching effects are being felt everywhere, not just in human communities. Thus, it is imperative to understand how the other citizens of the planet are responding to rapidly changing habitats so we can predict how they may deal with climate change in the future.
Birds, for example, deal with variations in temperature and rainfall by altering the timing of major life history events, particularly reproduction. Rainfall is particularly important for birds breeding in arid environments: increased rainfall is usually associated with increased offspring survival, whereas drought decreases offspring survival. But temperature is also important because periods of very hot weather not only lower offspring survival but also slow nestling growth rates. Thus, birds living in arid environments are particularly vulnerable to the effects of consistently drier and hotter weather created by climate change.
Some birds (and some mammals too) evolved strategies to help them successfully carve out a living in challenging environments, like deserts. One such life history strategy is cooperative breeding, which is where more than two adults raise a single brood of chicks to independence. The benefits of cooperative breeding are many, and include an earlier fledging age and more broods of chicks raised to independence per season, reduced costs of breeding for females, enhanced investment into eggs, increased recruitment of fledglings as future helpers and breeders, and the ability to raise overlapping broods.
Considering these benefits, it is not surprising to learn that cooperative breeding evolved in unpredictable environments, where sharing the workload across a group of adults helps protect against reproductive failure during periods of poor rainfall and reduced food availability, and permits greater investment in offspring. This implies that cooperative breeding increases the likelihood that birds can survive in challenging environments.
But what about periods of extreme heat? Heat can have substantially different effects on offspring survival at different developmental stages. Does group living protect cooperatively breeding animals — and especially their vulnerable offspring — from the effects of high temperatures?
How hot does it get in the Kalahari?
To get at this question, scientists from the FitzPatrick Institute of African Ornithology at the University of Cape Town, the University of Cambridge, and the University of Western Australia, collaborated to examine a comprehensive 15-year dataset of the southern pied babbler, Turdoides bicolor, to identify the relationships between temperature, rainfall, and group size on the survival of eggs, nestlings and fledglings.
The southern pied babbler is a medium-sized cooperatively breeding songbird endemic to the arid Kalahari savannah and desert located in southern Africa. Each group of birds numbers between 3-16 adults, and is centered around a dominant breeding pair of birds that produce more than 95% of all young hatched. Each brood ranges in size between two and five (but usually numbers three) offspring, and all group members help raise the young birds. Because the fledglings require care for an extended period of time, it is not uncommon to see chicks from several broods being raised simultaneously.
In the study site, the Kuruman River Reserve (where cooperatively breeding meerkats are also studied), all birds are marked as nestlings with a metal leg band engraved with a unique number and a unique combination of colored leg bands that make it easy to visually identify individuals from a distance. The birds have been habituated to being watched by people and are visited weekly during the breeding season to see who is in each group and to observe what they are doing, and particularly to record onset of breeding.
The Kalahari region is characterized by hot summers and periodic droughts, with extremely variable rainfall between years, although the frequency and severity of daily high temperatures have been increasing over the last 20 years. The average daily maximum temperatures at the study site averaged 34.7 ± 9.7°C (94.46 ± 17.46°F) and mean annual precipitation averaged 186.2 ± 87.5 mm (7.3 ± 3.4 inches) from 1995 to 2015.
The researchers predicted that high temperatures would reduce survival of the young birds whilst high rainfall would enhance their survival. The researchers also expected that larger groups of birds would be better at reducing the harmful effects of extreme climactic conditions on offspring.
Does cooperative breeding increase offspring survival when it’s exceptionally hot?
The data (489 breeding attempts by 50 groups of birds over 14 breeding seasons) confirmed the researchers’ prediction that eggs, nestlings and fledglings are far less likely to survive hot daytime air temperatures. On one hand, when the daily average air temperature was below 35.4°C (95.7°F), the age at hatch or fail remained unchanged, but when air temperatures rose above 35.4°C, the age at hatch/fail significantly declined. Further, high average daily air temperatures during the nestling period reduced survival to independence, suggesting that the harmful effects of high temperatures during early development continued to impact individual survival even after fledging.
But most disturbing was the finding that no young birds survived when daily maximum air temperatures exceeded 38°C (100°F) during any of the three development stages (egg, nestling, fledgling). This is troubling because daily air temperatures that meet or exceed 38°C are becoming increasingly common each summer. Further, these temperature data suggest that pied babblers’ reproduction is constrained both by available food resources and by their physiology at high temperatures and low rainfall, regardless of group size.
Rainfall was the strongest predictor of fledgling survival. This is not surprising because periods of increased rainfall promote greater food availability, which could enhance provisioning of fledglings, and also increase the likelihood that the young birds can find food for themselves. But drought is a fairly common event where the pied babblers live, so their ability to invest in increased breeding after a drought may be an important aspect of their post-drought recovery that supports their long-term persistence. Considering the likelihood that drought will only increase in the future, babblers will likely have fewer ‘recovery years’ available to them.
Surprisingly, the researchers did not find a direct relationship between group size and survival to independence. The authors noted in their paper that “[d]espite the intuitive appeal of the hypothesis that cooperative breeding may buffer against some of these effects, we found no evidence this will be the case in pied babblers.” Thus, the birds cannot rely on cooperative breeding to help them persist over the long-term, either.
In view of these findings, the warming temperatures and decreasing rainfall are causing increased chick mortality amongst pied babblers. This situation is particularly dire for birds breeding in tropical deserts. These findings also serve as a warning that all birds may be vulnerable to the effects of climate change because they lay eggs, which are stationary and must be maintained within strict temperature parameters in order to hatch. As this study clearly shows, nestlings and fledglings are also vulnerable to periods of excessive heat, which can kill them before they manage to achieve independence.
Climate change is a serious matter that already is having severe impacts on local weather patterns, glaciers and polar ice caps. According to the Intergovernmental Panel on Climate Change (IPCC), global surface temperatures are projected to increase by at least 1.5◦C, and more likely by 2◦C (and perhaps by twice that), by the end of this century. IPCC data predict that global warming will increase the frequency and duration of heat waves, too.
On one hand, 2◦C doesn’t sound like much, but for species, like pied babblers, that are already living at the edge of their thermal tolerances, this is critical.
The IPCC also predicts that global warming will further decrease annual precipitation in many mid-latitude and subtropical dry regions — which include the savannahs and deserts where pied babblers live. Birds can dehydrate and die after a few hours at 45oC (113oF).
Overall, the hotter and drier future that we are so carelessly creating does not look promising for pied babblers and other desert-dwelling birds.
Amanda R. Bourne, Susan J. Cunningham, Claire N. Spottiswoode and Amanda R. Ridley (2020). High temperatures drive offspring mortality in a cooperatively breeding bird, Proceedings of the Royal Society B: Biological Sciences, 287:20201140 | doi:10.1098/rspb.2020.1140
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