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On the Seychelles Islands in the Indian Ocean, little greenish-brown warblers mate for life—or at least a substantial part of it. Year after year, male and female bring the young chick insects to eat while vigorously defending the nest from skinks or predatory birds. These pairings are remarkably stable, lasting up to 15 years—a good chunk of their lifespans, which can reach 20 years—with just under 7 percent of warbler couples separating every year on average.
But scientists are finding that changes in climate can upset these harmonious partnerships. In a recent study in the Journal of Applied Ecology, a team of scientists discovered that in years with little rainfall or extremely high rainfall, divorce rates soared, sometimes more than doubling to 16 percent.
Researchers suspect these higher rates of divorce may be driven by the stress brought by food scarcity in years with harsher climate—a conclusion that may bode ill for the little birds’ partnerships in a world with increasingly variable rainfall. But scientists are still working out whether higher warbler divorce rates are necessarily a bad thing in the long run, or if they might even help the birds adapt to adversity. In the worst case, “it could have long-term effects on the population size,” says evolutionary biologist Hannah Dugdale of the University of Groningen in the Netherlands.
Since 1985, researchers have been carefully monitoring warblers on the Seychelles’ Cousin Island, catching them every year during the breeding season; each bird carries a metal ring with a unique identifier around its leg, along with a unique combination of colored rings so they can be identified with binoculars. The program is so thorough that if a bird isn’t sighted in a year, “we know that it has died,” Dugdale says. The bird couples are very territorial, so if one of them disappears from its territory and reappears in another, it’s clear that it has separated from its partner.
If divorce is causing warblers to break up with perfectly good mates, they may wind up single or with worse partners.
The reality is that males and females are sexually promiscuous and often cheat on one another, but once pairs begin to share a nest, the male will still stick with his female mate to care for her offspring—even when the chick is not his. The bird couples typically live together with helper birds—usually the female’s older offspring, both male and female, who help them in raising young. Female helper birds sometimes also breed and contribute an egg to the nest.
Scientists don’t know much about exactly how warbler divorces play out—when it happens, whether it’s the male or the female initiating the separation, or if it’s preceded by a period of strife or aggression. “Maybe one just leaves without saying anything. It could be a very sad affair,” says behavioral ecologist Augs Bentlage of the University of Groningen, a joint first author on the paper with his colleague Frigg Speelman, also at University of Groningen. After splitting up, some birds stay single while many find new partners.
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The warblers’ average annual divorce rate of 6.6 percent per year is comparable to that of other birds that tend to live out their lives in a single region but lower than that of many migratory species that have much higher annual divorce rates. For great reed warblers, for instance, it’s 85 percent. That’s partly because partners lose each other over long seasonal journeys and end up finding new mates along the way.
Seychelles warbler divorce rates were highest in years with unusually low rainfall, according to the team’s analysis of divorce data and local weather records. Bentlage suspects that dry weather reduces the vegetation that insects feed on and evaporates waters where the insects lay their eggs. Warbler couples would then struggle to find enough insects for their offspring, causing physiological stress. The birds may blame these difficulties on their choice of partner, the researchers hypothesize. “You might actually start to perceive your partner as being a low-quality partner,” Bentlage says. On top of this, male helper birds appear to be less likely to assist with chick-raising in years with low rainfall, while females didn’t change their behavior, according to some previous research by Dugdale and her colleagues.
The year of 1997, which saw exceptionally high rainfall in the Seychelles, also saw a spike in warbler divorce rates. Extreme rainfall can kill insects while strong winds can destroy bird nests. And if birds get wet, it’s harder for them to maintain optimum body temperatures. Parents would need to stay with chicks to help keep them warm, limiting their time for insect-gathering, causing stress. From low to high rainfall, “both extremes kind of have the same effect,” Bentlage says.
It’s still unclear what all this means for the warbler populations overall. Extreme years in the Seychelles are predicted to become more common as the global climate warms—a trend that is already making the birds less likely to attempt to breed and lay an egg as well as lowering fledgling survival rates, probably because there are fewer insects for them to eat.
Divorce could either help or hurt the birds during harsh years. Separation could counterintuitively be a good thing if the birds are indeed ditching partners who are bad at finding food, for instance. This might give them the chance to find ones who are better at it, giving future chicks a higher chance of survival. In line with this idea, some long-term research on black-browed albatross couples has found that when they divorce—which increases in years with warm sea surface temperatures—the females tend to find new partners with whom they’re more likely to produce chicks.
But, if divorce is causing warblers to break up with perfectly good mates, they may wind up single or with worse partners instead. This could, in turn, lower the number of chicks they make and, theoretically, over many generations, even increase extinction probability, Dugdale says.
Either way, it’s clear that the heating of the Earth’s atmosphere has not only obvious and direct impacts on animal survival, but also more subtle and complex influences on their lives, the decisions they make, and the relationships they cultivate with one another. 
Lead image: inimalGraphic / Shutterstock
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Katarina Zimmer
Posted on
Katarina Zimmer is a freelance science and environment journalist.
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