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Michela N. Dumas, Christoph M. Meier, Pierre Bize, and Julien G. A. Martin: Read the article
Stay or go? Dumas et al. found that mate switching behaviours ('divorce') in the Alpine swift likely follow a win-stay lose-switch strategy, and that females may have more to gain by going
While humans struggle to make relationships last—with nearly half of first marriages ending in divorce—Alpine swifts seem to have cracked the code. With a remarkably low divorce rate of 16.6%, these birds are relationship goals. Dumas et al. use anthropomorphic terms like ‘divorce,’ ‘widow,’ and ‘fidelity’ to describe alpine swifts’ pairing behavior, offering a window into their surprisingly complex love lives. Birds, in general, are among the most faithful creatures in the animal kingdom. While monogamy is relatively rare in most animals—such as amphibians (frogs, salamanders, and caecilians), non-avian reptiles (lizards, snakes, and turtles), and mammals (bears, tigers, lions, etc.)—it is much more common in birds. In fact, an estimated 80-90% of bird species practice monogamy, and alpine swifts are no exception to this trend.
Finding a good partner can be challenging. If your best friend recently went through a breakup, you might feel inclined to reassure them by saying there are plenty of options out there. However, only those who have ventured into the world of dating apps truly understand how difficult it can be to find someone genuinely compatible, despite the abundance of choices. Even when it seems like we’ve finally found ‘the one,’ maintaining a secure and lasting relationship can be a struggle. Often, staying in an imperfect relationship might feel easier than starting over. By contrast, Alpine swifts usually choose a partner and stick to them. This strategy has evolved to reduce the costs of finding and securing a new partner. However, there are occasionally ‘divorces,’ where they decide to venture into the bird dating world even if both partners are still alive.
If staying in the same relationship seems to work for most pairs, why do some birds divorce? There can be multiple reasons, and no bird should have to settle for an unsuitable partner. Some birds divorce with the hope of finding a more compatible or higher-quality partner. Others divorce to get access to a higher-quality territory (better house in a better neighborhood that their ex could not provide). Putting yourself out there can be risky, so for divorce to be worth it, the benefits need to outweigh the costs of finding that new and improved partner. However, relationships can end for reasons outside of our control, too, such as the displacement of a partner by a more aggressive bird or coming home after a long migration only to find that your partner didn’t wait for your return. Besides keeping the same partner, there is also the matter of keeping the same nest. Building a beautiful home takes a lot of resources, and moving is expensive, so nest retention can be very advantageous.
Perhaps one of the secrets to a long, successful relationship is the equal division of household chores. Alpine swifts have bi-parental care, which means that both males and females spend time at home and with the kids. Both parents build and maintain the nest, incubate the eggs, and feed their chicks. Dumas et al. tracked the lives of hundreds of Alpine swifts nesting in Switzerland from hatching to death for three decades (1991–2021). To better understand the consequences and predictors of divorce, they categorized each breeding event as divorced, faithful, or widowed, depending on how the birds paired compared to the previous year for a total of 2,260 pairs! Although ~50% of birds kept the same partner their whole lives, some birds went through as many as five divorces. There were a few cases where a pair divorced, paired with someone else, divorced again, and came back to their exes, showing that the grass is not always greener on the other side. Widowed birds would re-pair between one and five times after their original loss. One male was an outlier who re-paired eight times! Either he never mourned, or maybe he was in so much pain that he had to re-pair 8 times to forget his first love.
In terms of real estate, in all the tracked pairings, 49% stayed with the same partner and same nest. So, who keeps the house after the divorce? Apparently, both sexes usually leave the nest—no one wants the painful memories. However, after divorce, it was most often the males that would keep the nest (20%), compared to females (14%). As for the causes and predictors of divorce, the authors found that pairs that had fewer chicks were more likely to divorce. For humans, having another baby might not save a marriage. But for Alpine swifts, having more offspring might just do the trick. Another factor that influenced divorce was the age of the birds, as younger individuals, both male and female, were more likely to divorce. It does not always work out to stick with your high school sweetheart.
Regardless of the why, divorce always takes a toll. Divorced birds had lower reproductive success than widowed or faithful birds. Alpine swifts who divorced seemed to lose time finding a new partner and, therefore, laid their eggs later in the season. They needed to get to know their new partner and so managed to lay fewer eggs and had fewer surviving chicks than widowed or faithful birds. But, compared to their own stats from the previous year, divorcées had an improvement in their reproductive success. Good for them… even if a new relationship didn’t fix everything, this teaches us that we should never compare our own journey with others.
The authors wanted to calculate the heritability of divorce: If your parents got divorced, how likely is it that you will get divorced too? They did not find it to be very heritable, which might mean the behavior is more environmental than genetic. No worries, kids of divorced parents, this means divorce is not strongly genetically determined. And who to blame for the divorce? The authors could not determine who initiated the divorce in each pair. Measuring that would be extremely challenging. My therapist would say that most of the time, there is blame to share if a relationship does not work out. For more details on this fascinating study, check out Dumas et al.’s full publication.
Diana Aguilar Gómez is a Chancellor's Postdoctoral Fellow in the Ecology and Evolutionary Biology Department at UCLA. She is interested in non-model organism genomics to answer questions about adaptation and conservation. She has worked with frogs, lizards, pumas, humans, and whales. In her free time, Diana enjoys dancing, cooking, knitting, and being outside.
