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Albatrosses divorce more frequently when ocean waters warm

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Birds match the bill when it comes to fidelity: Over 90% of all bird species are monogamous and — for the most part — faithful, probably none more so than the majestic albatross. Albatross couples seldom split up, preferring to procreate with the same mate year after year. However, when ocean temperatures are warmer than usual, more birds split off, according to a recent study.

Researchers report November 24 in Proceedings of the Royal Society B that when the water was warmer than usual, the divorce rate among albatrosses is one of the Falkland Islands jumped to about 8%, up from less than 4% on average. It's the first proof that the environment, rather than only breeding failure, influences divorce in wild birds. The study discovered that during warmer years, even some bred females successfully abandoned their partners.

The findings show that as the environment changes as a result of human activity, increasing divorce rates in albatrosses and possibly other socially monogamous animals may be an "overlooked consequence," according to the researchers.

Albatrosses can live for decades, spending years at sea foraging for food and coming to land only to breed. Couples that stay together benefit from familiarity and increased coordination, which is beneficial while raising children. According to Francesco Ventura, a conservation biologist at the University of Lisbon in Portugal, this stability is especially vital in dynamic marine ecosystems.

However, if breeding fails, many birds — primarily females — abandon their partner in search of greater luck elsewhere (SN: 3/7/98). Breeding is more likely to fail in years with more difficult conditions, with subsequent impacts on divorce rates. Ventura wanted to know if the environment had a direct impact on the rate of divorce, regardless of how well the breeding went.

Ventura and his colleagues examined data from a large colony of black-browed albatrosses (Thalassarche melanophris) living on New Island in the Falkland Islands from 2004 to 2019. The researchers tracked avian breakups and roughly 2,900 breeding attempts in 424 females. After accounting for historical breeding success in individual pairs, the researchers examined if environmental variables had any further impact on pairings.

Breeding failure, particularly early on, was still the most common cause of divorce: Each female lays just one egg, and birds whose eggs did not hatch were more than five times more likely to depart from their spouses than those who succeeded, or whose hatched offspring did not survive. In some years, the divorce rate was less than 1%.

However, this percentage climbed in lockstep with average water temperatures, peaking at 7.7 per cent in 2017 when the water was at its warmest. The team's analyses found that rising temperatures were related to the likelihood of divorce. Surprisingly, females in successful breeding pairs were more susceptible to the harsher environment than males or females that did not breed or failed to breed. Divorce rates fell when ocean temperatures fell again in 2018 and 2019.

Warmer water equals fewer nutrients, so some birds may be filling up at sea for longer periods, delaying their return to the colony or arriving bedraggled and unappealing. Breakups might occur when members of pairs return at separate times (SN: 10/6/04).

Furthermore, worsening conditions one year may cause stress-related chemicals to be released in the birds, which can influence mate selection. According to the researchers, a bird may mistakenly ascribe its stress to its companion rather than the tougher environment, causing it to separate even if hatching was successful.

Separation may become less effective as a result of such misunderstanding between cues and reality, according to Antica Culina, an evolutionary ecologist from the Netherlands Institute of Ecology in Wageningen who was not involved in the study. If animals split for the wrong reasons and perform poorly the following season, this might lead to poorer overall breeding success and possibly population decline.

According to the researchers, similar patterns could be seen in other socially monogamous creatures, including mammals. "Imagine a population with a very low number of breeding couples... this may have even more devastating consequences," Ventura says.