It’s no surprise how the lovebirds got their name and how that name became slang for an affectionate human couple. Pairs of these African parrots feed, hug, groom each other and become distressed when separated. His behavior is even better summed up by his Spanish name: inseparable.

Lovebirds are unusual not only for their cheeky PDA, but because they choose a partner for life. While more than 90 percent of birds mate in the practice of “social monogamy,” most choose a new partner every year or every few years. Besides lovebirds and a few other parrots, only a few birds mate for life, including swans, eagles (shout out to our favorite bald eagle team: Jackie and Shadow), and albatrosses. Lifelong mating is even less common in non-avian animals.

You might expect it to be more practical to keep the same mate year after year than to have to find a new one each breeding season. But it is not always the most effective plan, especially if the animal pair does not stay together all year round.

From an evolutionary point of view, staying with a single partner is not best for all animals. Although some animals have lifelong partners, most prefer different reproduction strategies. And far from being the perfect image of two lovebird soulmates, even monogamous animal relationships include just as many complications as human ones.

Mating for life has many advantages

In birds, “social monogamy is linked to biparental care: the male and female care for the offspring together,” says Dr. Bart Kempenaers, Director of Ornithology at the Max Planck Institute for Biological Intelligence in Seewiesen, Germany. For bird parents, “it is advantageous to stay together because you have to coordinate this care. With more experience raising together, this coordination might be easier.”

Teamwork for long-term couples is not limited to co-parenting. Kempenaers points out that some birds, like cardinals, associate throughout the year, not just during the breeding season. When there are no offspring to raise, the male and female cardinals defend their territory together.

Mammals and other animals can also derive various benefits from a lifelong association. In addition to sharing parental care, wolves, coyotes, and foxes hunt and defend territory with their partners, while beaver pairs maintain their dams and lodges together. Breeding pairs of French angelfish (one of the only fish that mate for life) do not care at all about their young, instead staying together to patrol their feeding grounds.

What makes some birds more likely to stay together?

In birds, Kempenaers identifies longevity as an important influence on whether a species mates for life. “To stay together, both partners have to live until the next breeding season,” he says. And since “larger birds, like albatrosses, live much longer than a small bird like a titmouse” on average, Kempenaers says, birds that mate for life are mostly large-bodied and long-lived. Albatrosses, which mate for life, can live more than 50 years; Chickadees, which do not, live only two or three years in the wild.

Small animals also have more predators to deal with than large ones, reducing their chances of surviving from one breeding season to the next. Many of the birds and mammals that mate for life, such as eagles and wolves, have few natural predators.

There are also other factors that influence mating habits. For two pairs to stay together long-term, they must be able to consistently meet each breeding season. Birds that mate for life are generally those that stay together year-round or return to the same places at the same time each year. They may even use the same nesting site every year, what biologists call “site fidelity.”

Pairs of birds can “divorce”

The breakup of a couple is often called a “divorce,” but Kempenaers warns that this term “suggests that it is a decision for one or both partners to stay together or not, and that is not necessarily the case.”

Some birds that mate for life, such as cranes and swans, migrate together in family units. However, in most migratory birds, pairs travel separately. This may lead to birds choosing a new partner out of necessity rather than incompatibility with the previous partner.

“If one of the partners returns [from migration] but the other partner has not yet returned, so this individual faces a dilemma,” says Kempenaers.

“It doesn’t know if its mate is still alive. The breeding season is underway and it is ready to breed. Therefore, this bird might decide to mate with another individual rather than wait for the mate, which may never appear.” Even if last year’s partner arrives late, Kempenaers notes, the one who arrived first will usually stay with their new partner.

For birds, taking advantage of the opportunity to reproduce comes before human notions of fidelity. One study of mute swans found that their low “divorce” rate was three times higher in pairs that did not successfully reproduce than in pairs that did (nine percent versus three percent).

Animals also “deceive” each other

Social monogamy in animals does not necessarily mean sexual monogamy. Even for species that mate for life, there may be survival benefits to associating with one partner while continuing to mate and have offspring with others.

This behavior is common in all bird species. In some cases, Kempenaers says, the “social father” cares for the offspring, “but is not the father of any of them” genetically, or at least some of them. A study of black swan pairs found at least one baby fathered by a different male in up to 40 percent of nests. Similar behavior has been observed in socially monogamous mammals, such as wolves and gibbons.

“If you’re asking why this happens or what the advantages are, it’s always important to distinguish between the male side and the female side,” Kempenaers says. Males who mate with many females could be trying to father as many offspring as possible. However, Kempenaers says, “it is more difficult to understand” why females often take the initiative to leave their nesting territory to associate with neighboring males.

This could ensure that the female’s own genes are passed on, in case her nesting partner is infertile. It might even require additional parental care from other males, as has been observed in some birds, such as the blue tit. But the exact factors behind this behavior remain a mystery.

There is still a lot we don’t know about animals’ mating habits.

Kempenaers explains that scientists are still working to define what better cooperation between partners that stay together long-term really looks like. Other existing questions include how different mating systems evolve and why they can vary so much, even between species that have a lot in common.

Kempenaers was part of a 2023 study of the Alaskan breeding grounds of a shorebird called the long-billed dowser. Dowsers are relatively large, long-lived birds that practice social monogamy. “Both the male and female are essential for reproduction,” says Kempenaers, and they share tasks such as incubating eggs.

A bird with these characteristics could be expected to return to the same partner and nesting site, at least for several years in a row. This is what similar birds do in the same environment as dowsers. However, dowsers choose a new partner and a new place to nest each year. “We found this really disconcerting,” says Kempenaers. “If everyone else returns to the same place, why don’t dowsers do this?”

To find out, researchers attached satellite trackers to the dowsers and followed their movements throughout the year. They discovered that “females leave the breeding grounds much earlier than males and spend the winter in different places,” says Kempenaers. The most important thing is that once they have mated, “they never meet again.” But why dowsers behave this way when other socially monogamous birds in the same environment do not “remains an enigma,” he adds.

In nature, fidelity has benefits for some, but not for others, although it is not always perfectly clear what those benefits are. And while it’s tempting to see animal relationships in human terms, monogamy in the animal kingdom often looks very different from human romance.

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