‘Divorce’ Rates of Seychelles Warblers Linked to Rainfall Fluctuations During Breeding Season

The amount of rain that falls affects our environment in various ways, from river flow to the availability of freshwater, but it can also shape the diversity and distribution of ecosystems within different regions of the globe.

A new long-term study of Seychelles warblers (Acrocephalus sechellensis) on Cousin Island has revealed that rainfall during the breeding season can affect “divorce” rates between the birds, highlighting the complex effects climate change can have on animal reproduction and conservation.

The study, conducted on a closed Seychelles warbler population, uncovered major findings on how rainfall and other environmental factors can negatively affect the stability of pair bonds in birds, a press release from Macquarie University said.

“This instability can occur because of the effects rainfall patterns have on the ecological environment of the warblers, where rainfall influences food availability, and nest/habitat conditions, all of which can affect the health of the birds before and during the breeding season and affect their ability to produce offspring,” A. A. Bentlage, a researcher in the School of Biological Sciences at Australia’s Macquarie University, told EcoWatch in an email.

Seychelles warbler pairs tend to stay together in favorable weather. Charli Davies

For species like the Seychelles warbler, who are socially monogamous, “divorce” refers to the breaking up of a pair bond when both individuals are still living. The mating strategy of pair bonding, observed in various species, has often been associated with lower reproductive success, or the number of offspring they produced as a pair.

Social monogamy implies that birds form a pair bond and spend many years — even lifetimes — with the same partner. Birds in these partnerships typically live and raise their young together, but they are not always sexually monogamous.

The relationship between divorce and environmental factors — especially fluctuations in climate — is still poorly understood.

“We analyzed 16 years of data from a longitudinal dataset to examine the influence of rainfall on the prevalence of divorce and the possible underlying mechanisms. First, we performed climate window analyses to identify the temporal windows of rainfall that best predict reproductive success and divorce. Then, we tested the effects of these temporal windows of rainfall on reproductive success and divorce and the influence of reproductive success on divorce while controlling for covariates using generalized linear mixed models,” Bentlage told EcoWatch.

The study, “Rainfall is associated with divorce in the socially monogamous Seychelles warbler,” was published in the Journal of Animal Ecology.

“We… found a complex, non-linear relationship between rainfall patterns and divorce rates, with divorce being more likely during years of both low and high rainfall,” said Frigg Speelman, a member of the science and engineering faculty at Macquarie University, in the press release. “This relationship was notably influenced by an extreme climatic event — the 1997 El Niño-induced rainfall spike.”

The researchers found a correlation between the extreme rainfall associated with the 1997 El Niño event and divorce rates of the Seychelles warbler. However, when they excluded this exceptional event from their analysis, it became clearer that there was indeed a negative relationship between rainfall amounts and the birds’ divorce rates.

The study did not find any direct link between the reproductive success of the birds and divorce likelihood, however, which suggested that there may be other factors influencing partnership stability.

Bentlage said rainfall extremes have a significant effect on the food sources and habitats of the Seychelles warbler.

“Extreme rainfall can greatly affect the ecological environment of our study population. For example, low rainfall significantly diminishes food availability by disrupting the development of insects that often lay their eggs in water, and heavy rainfall can destroy habitats/nests and make maintaining optimal body temperatures difficult for birds. Therefore, both rainfall extremes can impact the birds’ health and make raising offspring difficult,” Bentlage told EcoWatch.

The researchers found that the highest divorce rates occurred in years with extremely high or low rainfall occurring during breeding season, possibly affecting how the birds viewed their partners.

“Whereas divorce in many species is often correlated with poor reproductive success, our study found no evidence for this in the Seychelles warbler, suggesting that other, more complex factors may be involved. For example, the aforementioned rainfall effects may cause an increase in stress for the birds, increasing the instability of their partnerships as they associate the stressful period – that was caused by extreme weather – with their partner. Thus, extreme rainfall could misinform these birds about the quality of their partner, causing them to reconsider their choice of partner and separate,” Bentlage said.

The study builds on mounting evidence demonstrating how environmental factors — especially those associated with climate change — can have a direct effect on the reproductive strategies and social dynamics of wildlife.

The Seychelles warbler is a native species of the Seychelles islands. The rare birds are already facing a number of conservation challenges, which make the study particularly relevant. The findings of the research suggest the species could experience further stress to population stability and reproductive success due to environmental changes.

“As climate change intensifies, it is critical to understand how fluctuations in environmental conditions, such as rainfall, affect the stability of socially monogamous species,” Speelman said in the press release. “This research not only enhances our understanding of animal behavior in the face of climate variability but also provides valuable insights that could inform conservation efforts for species vulnerable to the effects of climate change.”

What other environmental changes and stressors are Seychelles warblers facing due to climate change?

“The Seychelles warblers are a great conservation success story as they were once on the brink of extinction, with just 26 individuals left in the world. After Cousin Island and the surrounding sea became a nature reserve in the 1960s/70s the population bounced back to stable numbers. Thus, they are doing quite well at the moment. Nevertheless, as a result of climate change, extreme rainfall events are predicted to become more prevalent and alongside them, other stressors such as extreme winds may also become more of an issue. Wind can influence territory quality on Cousin because wind-driven onshore salt spray negatively affects vegetation. Thus, habitat destruction that influences territory quality/food abundance may become more prevalent in a changing world,” Bentlage told EcoWatch.

Bentlage said the research team does not know how divorce driven by rainfall extremes will impact Seychelles warblers in the long-term, but once that is understood it could help improve future conservation efforts.

“We do not yet understand whether rainfall-driven divorce in the Seychelles warbler is adaptive, maladaptive, or has no effect on short/long-term reproductive fitness. As we discuss in our paper, it is essential to investigate the fitness consequences of divorce in the context of these extreme environmental effects to understand to what extent plasticity in breeding behavior may enable socially monogamous species to adapt to a rapidly changing world,” Bentlage explained. “Once that is understood, combined with our current knowledge of the negative effects of extreme rainfall on reproductive success, our findings can greatly inform population modeling and monitoring, such as using environmental indicators as early warning signals for potential declines in population stability. Also, for some species, such knowledge may even inform habitat management and conservation approaches that have specific climate change adaptations in mind. For example, efforts could be made to protect habitats or create refuges that provide optimal conditions during poor climatic periods.”