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It is commonly believed that over 90% of bird species have one mate over one breeding season or longer, but it turns out that some birds switch to a different partner for another breeding season even if their mate is still alive - in a behavior labeled “divorce”.

Researchers in China and Germany concluded that there are two key factors that lead to a divorce across a broad range of bird species - male promiscuity and long-distance migrations.

Published in the journal Proceedings of the Royal Society B, the study describes how previously published data regarding divorce rates for 232 species of birds with mortality data and migration distances proved their thesis.

Males and females of each species were given a separate “promiscuity score” according to published information about the birds’ behavior. It was then discovered that species with high divorce rates tend to be closely related to each other, and a similar pattern was seen for male promiscuity.

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“For instance, plovers, swallows, martins, orioles, and blackbirds had both high divorce rates and male promiscuity, whereas petrels, albatrosses, geese, and swans had low divorce rates and male promiscuity,” the team said.

Climate may play a role

Surprisingly, while higher male promiscuity was linked to higher divorce rates, this was not the case for female promiscuity.

Dr Zitan Song, a co-author of the research at Max Planck Institute of Animal Behavior in Germany, explained that “When a male bird is promiscuous, it is often perceived as a reduction of this commitment, as his attention and resources are divided among several females", adding: "This can make him less attractive as a partner, and thus more likely to be ‘divorced’ in the next breeding season. Conversely, a male can augment his fitness by mating with multiple females".

Still, Song suggested female promiscuity may not cause the same consequences because uncertainty about the offsprings' paternity could result in increased male involvement in parental care.

Longer migration distances also contributed to a higher divorce rate.

“After migrating, pairs may arrive at their destination asynchronously, leading to a situation where the early arrival might mate with a different partner, resulting in a ‘divorce’. Migration could also lead to pairs landing in different breeding sites, thereby causing ‘divorce’ due to accidental loss. This effect intensifies with increasing migration distance,” said Song. 

“Divorce can aid in facilitating immediate breeding upon arrival, rather than waiting for a previous partner,” Song said.

The team says that divorce in birds may not insinuate a strategy to boost an individual’s fitness, or a response to factors such as migration, but instead could simultaneously be impacted by both.

Dr Samantha Patrick, an expert in marine biology at the University of Liverpool, who was not involved with the research, agreed with the results.

“Perhaps given my own current research interests, I find the results linking migration asynchrony to divorce [most interesting],” she said. “As the climate becomes more unpredictable migration timings may become more variable and this paper suggests that across species this could increase divorce rates.”