This blog post is provided by Frigg Speelman and tells the #StoryBehindThePaper for the paper “Rainfall is associated with divorce in the socially monogamous Seychelles warbler”, which was recently published in the Journal of Animal Ecology. In their study, Speelman and colleagues analysed a 16-year dataset and found that probability of divorce for Seychelles warblers was closely related to the amount of rainfall received during the breeding season

How extreme weather is impacting the romantic lives of birds

The natural world is full of surprises, and one of the more fascinating behaviours observed in many animal species is divorce — the termination of a pair bond while both members are still alive. While the idea of divorce is often associated with human relationships, many socially monogamous species exhibit this same behaviour, particularly when they experience poor reproductive success. Now, a new study delves into the previously underexplored link between environmental variability and divorce rates in the animal kingdom. Focusing on the Seychelles warbler, a small passerine bird, we reveal how fluctuating weather patterns affect these birds’ ability to maintain stable partnerships.

Why break up with your partner?

Using an extensive 16-year dataset, we studied the Seychelles warbler (Acrocephalus sechellensis), a bird species endemic to the Seychelles islands. Seychelles warblers are socially monogamous, meaning pairs typically stay together for long periods, but divorce, when it occurs, can be a strategy used to escape poorly performing partnerships and seek a better-fitting partner elsewhere. Divorce is relatively common in the Seychelles warbler, ranging from 1% to 16% annually. This could be a beneficial strategy, allowing birds to ‘correct’ for a poor initial choice of a partner, thereby increasing their chances of reproducing successfully. However, many factors could misinform these birds, meaning they divorce while this does not benefit them per se. We delved deeper into one of these potential components – rainfall.

Rainfall: A Key Factor in Divorce

In this study, we investigated how rainfall during specific “climate windows” correlated with both divorce rates and reproductive success. What we found was quite compelling: the probability of divorce was closely associated with the amount of rainfall the island experienced in the seven months leading up to and during the breeding season. The relationship between rainfall and divorce wasn’t linear, but rather quadratic — meaning that divorce rates spiked both when rainfall was too low and when it was too high.

One major finding was tied to the 1997 super El Niño event, which caused exceptionally heavy rainfall that year (1430 mm versus an average of 884 mm). During this year, the overall divorce rates were extremely high (15.3%) suggesting that moderate rainfall promotes pair stability, whereas extremes in either direction led to higher divorce rates. When we excluded the super El Niño year, we detected a negative linear relationship between rainfall and divorce. By analysing these patterns, we were able to shed light on the broader implications of environmental changes on the behaviour and relationship dynamics of these birds. As we expect these extreme events to become more common with climate change, this may be concerning for the stability of romantic partnerships in wild animals.

Reproductive Success and Divorce: A Complex Relationship

While divorce in many species is often directly linked to poor reproductive success, the study found no significant correlation between these two factors in the Seychelles warbler. Even though the temporal window of rainfall that predicted divorce also influenced reproductive success, the two were not significantly correlated. This suggests that other, more complex factors may be at play; for example, extreme weather affects the physical environment, such as food availability or nesting conditions. This may increase the stress experienced by the birds and therefore increase instability in their partnerships, but at the same time, it may not necessarily result in reduced reproductive success.

The Broader Implications of Climate Change

As climate change leads to more frequent and intense weather events, the findings from this study have broader implications for understanding how environmental factors can influence the romantic lives of birds, and even more broadly the social behaviours in the animal kingdom. Birds like the Seychelles warbler are particularly vulnerable to these changes because, like many other species, their reproductive strategies are tightly linked to environmental conditions. Increased divorce due to extreme rainfall patterns might ultimately impact the stability of entire populations and their ability to reproduce and survive successfully.

As we continue to face the challenges posed by climate change, studies like this are essential. They not only deepen our understanding of the natural world but also offer vital information for conservationists working to protect species that are highly sensitive to environmental fluctuations and have nowhere else to go in the event of climate change. As climate patterns become increasingly erratic, we will likely see more dramatic shifts in the social structures of many species, with implications not only for their survival but for the ecosystems they inhabit.

Read the paper

Read the full paper here: https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2656.14216

Author: Frigg Speelman

I am a behavioural ecologist studying pair-bonds in birds. My research aims to provide insights into why birds are so often sticking to the same partner. I use long-term multigenerational data of entire populations to monitor how individuals spend their life with a partner as well as movement data of pair-bonded birds using novel automated fine-scale radiotracking technology to see how much time they spend together as a pair.