MELBOURNE (NYTIMES) – Albatrosses usually mate for life, making them among the most monogamous creatures on the planet. But climate change may be driving more of the birds to “divorce”, a study published last week by New Zealand’s Royal Society says.

The study of 15,500 breeding pairs of black-browed albatrosses on New Island in the Falklands used data spanning 15 years. The researchers, led by Francesco Ventura of the University of Lisbon, found that the divorce rate among the birds, which averaged 3.7 per cent over that period, increased in years in which the ocean was warmest. In 2017, it rose to 7.7 per cent.

Albatross divorce is typically very rare. The most common trigger for permanent separation is an inability to successfully fledge a chick, the report noted. In the years that the sea was unusually warm, the albatrosses were more likely both to struggle with fertility and to divorce – the technical term used by the researchers – foreshadowing a worrisome trend for seabird populations in general as temperatures rise globally.

“Increasing sea surface temperature led to an increase in divorce,” Ventura, a conservation biologist, said in an interview.

But even after the models factored in higher breeding failure in warmer years, that by itself did not explain the rise in divorce rates, the researchers found.

“We see there is still something that is left unexplained,” Ventura said.

The large sea birds are found across the Southern Hemisphere, in countries like New Zealand, and off the coast of Argentina. They are known for their expansive travels, wingspan of up to 3.4 metres and long lives. They can survive for decades. The black-browed albatrosses take their name from the swooping, sooty brows that give them an expression of perennial irritation.

Albatrosses in partnerships spend most of the year apart, reuniting each season to raise chicks together. The male typically arrives first on land, where he waits for his partner and tends to their nest.

“It’s pretty obvious they love each other,” said Graeme Elliott, an albatross expert at New Zealand’s Department of Conservation who was not involved in the New Island study.

“After you’ve been watching albatrosses for 30, 40 years, you can kind of spot it. They do all this stuff that we think’s important – human emotion stuff, you know – greeting the long-lost mate, and they love each other, and they’re going to have a baby. It’s wonderful.”

The birds usually return to the same partner each breeding season. The pairs perform a dance of reunion that becomes more synchronised over the years.

“They increase the quality of the performance with the years – first a bit awkward, and then, as time goes by, they get better and better and better,” Ventura said.

The stress of warmer seas appears to disrupt that delicate balance, especially if the birds arrive for the breeding season late or in poorer health after having flown farther to find food.

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“We expect cooler waters to be associated with more nutrient-rich and more resource-rich conditions, whereas warmer waters are resource-poor conditions,” Ventura said.

Some albatrosses in the population studied ended successful unions and recoupled with a different albatross, the researchers found. (Females, who have an easier time finding a new mate, tend to be the instigators of permanent separations.)

“After a difficult resource-poor breeding season, the greater effort and higher breeding investment can lead stressed females to disrupt the bond with their previous mate and look for a new one, even if previously successful,” the researchers wrote.

Elliott, the New Zealand albatross expert, said the study’s finding “doesn’t surprise me that much”. Researchers have noticed demographic changes among birds elsewhere as fish populations have declined, he said.

The number of albatrosses on the remote Antipodes Islands, about 850km south of New Zealand, has declined by two-thirds over the past 15 years, according to the New Zealand Department of Conservation.

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Climate change is a factor: Female birds have travelled well off course in search of harder-to-find food, drawing them into deadly contact with fishing boats and leading to significant population imbalance, Elliott said.

That has prompted desperate decision-making by male albatrosses who find themselves single, he said. Male-male pairs now make up 2 per cent to 5 per cent of the bird population on the island, echoing a pattern of same-sex mating behaviour across many species. The behaviour, with its lack of a clear evolutionary advantage, generally continues to stump researchers.

“We’ve got 1 1/2 to 2 times as many males as females on the island now,” Elliott said. “We’ve been getting these male-male pairs forming – the males can’t find mates, and after a while, they decide other males are better than nothing at all.”

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