These birds form a trio, but probably not a group

Cranes have a reputation for romance. The birds live in faithful pairs, dancing and defending their territory together. As the intruders approach, the birds raise their beaks and sing a loud song in one voice.

In India, the sarus crane – with a purple head and as tall as an adult – is famous for its monogamy. “When one of the birds dies, local mythology says that the other bird is lazy with grief,” said KS Gopi Sundar, a scientist at the Indian Foundation for Conservation of Nature. “The truth, of course, is a little different.”

Dr. Sundar finds that pairs of sarus cranes sometimes leave a third bird to join them. He described the behavior last month in the journal Ecology. Living as a trio – alas, not much – can help birds raise young in poor conditions, one of which can behave a bit like an Au Pair. The birds even turn their iconic duet into a song for three.

“Dr. Sundar first saw a trio of sarus in 1999. When I mentioned it to US experts, they smiled and patted me on the head,” he said. But he was not ready to give up the idea. He followed this trio for the next 16 years.

Starting in 2011, he also trained field assistants (usually local farmers) to monitor sarus cranes. After collecting data in 2020, Dr. Sundar and Swati Kitur, a colleague of the foundation, dug into this database to search for trios.

Observers spotted 193 trios among more than 11,500 sightings of cranes. “So trios are definitely a rarity,” Dr. Sundar said. Some included a man and two women; some were the opposite.

Suhridam Roy, a graduate student at the foundation, visited four of these trios and released recordings of other pairs of cranes singing their territorial duets. In response, each trio made its own synchronized call. Scientists called it triet.

The data do not reveal how many chicks raised these trios and how long they stayed together. But 16 years of watching this original trio have given some hints about their family dynamics.

These cranes lived in a low-quality habitat, where the lack of wetlands would most likely make it difficult for a typical duo to raise young ones, Dr Sundar said.

But in a group of three, the result turned out to be better. Each year, one adult of this trio, a female, disappears while the other two nest and lay eggs. “It wasn’t a threesome,” Dr. Sundar said. Only two of the three animals mated each season.

But when the resulting chick or chicks were about a month old or as soon as the nest failed, the absent female reappeared. If there were chicks, she helped feed them. And working together, the three cranes raised chickens almost every year.

“Finding new behaviors like this in a system we all thought were monogamous for a long time is super interesting,” said Sahas Barw, an evolutionary ecologist at the Smithsonian Institution’s National Museum of Natural History in Washington, DC.

And the study raises many questions, he said. Most importantly, “Who is this third bird?”

For some species of birds, including the Florida bush jay and the Seychelles cinderella, the adult offspring are often left behind to form a trio with their parents and help raise their siblings, Dr. Barv said.

But Dr. Sundar believes the trio of sarus cranes are unlikely to include a grown chick, based on other research he has done. However, he noted that the third adult could be connected in another way. Sharing some genes with the chicken may help explain how this system evolved.

However, if the third adult is not related – and if he is not allowed to mate – what benefit does he get from living in a trio?

“The only benefit we could think of for the third bird is that it gets practice,” said Dr. Sundar. The helper can learn how to protect his home and feed the chicks. At least one trio the researchers are observing includes a very young man.

The researchers also found that the trio were more common in unwanted habitats. Dr. Sundar believes that unification can be an adaptation to bad circumstances.

Team parenting appears in the animal kingdom. Species of monkeys, mongooses, spiders, insects, birds and fish are involved in cooperative breeding. So do people. But so far no taps have been known for parents in teams.

“These are challenging assumptions we have about this family of birds,” said Anne Lacey, senior program manager for North America at the International Crane Foundation.

Ms. Lacey said she and her colleagues have never seen a trio among North American cranes, but added: “Could it happen when we just don’t watch? Absolutely. ”

Dr. Sundar plans to use genetics to find out if Sarus’ helpers are related. One question he does not intend to ask, however, is whether the helper is ever the real parent of the chick. In other words, is the sarus crane really monogamous?

“These birds are preserved because of the mythology that they are with each other all the time and that they are faithful,” he said.

Learning that some cranes deviate from their mates, Dr. Sundar said, risks disrupting the human-bird bond. “Why are we destroying this mythology for statistics and for a scientific article?” He said.

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