Cranes have a reputation for being romantic. The birds live in faithful pairs, dancing and defending their territory together. As the intruders approach, the birds raise their beaks and make a loud song with a voice.
In India, the crimson sarus, with its crimson head and as tall as an adult human, is famous for its monogamy. “When one of the birds dies, local mythology is that the other bird is moving away from grief,” said KS Gopi Sundar, a scientist at the Foundation for Nature Conservation in India. “The truth is, of course, a little different.”
Dr. Sundar discovered that pairs of sarus cranes occasionally let a third bird join. He described the behavior last month in the journal Ecology. Living as a trio, unfortunately, not entirely, can help birds raise pups in poor condition, with one that may behave a bit like an au pair. The birds even turn their signed duet into a song for three.
Dr. Sundar first saw a trio of sarus cranes in 1999. “When I told US experts, they smiled at me and hit me on the head,” he said. But he was unwilling to let go of the idea. He followed this trio for the next 16 years.
From 2011, he also trained field assistants (usually local farmers) to supervise sarus cranes. After gathering data until 2020, Dr. Sundar and Swati Kittur, a colleague of the foundation, dug into this database to look for trios.
Observers had detected 193 trios among more than 11,500 crane sightings. “So trios are definitely weird,” Dr. Sundar said. Some included a male and two females; some were the other way around.
Suhridam Roy, a graduate student of the foundation, visited four of these trios and played recordings of other pairs of cranes singing their territorial duets. In response, each trio made their own synchronized call. Scientists called it triet.
The data does not reveal how many chicks these trios raised or how long they were together. But 16 years watching that original trio gave some clues about their family dynamics.
These cranes lived in low-quality habitat, where the lack of wetlands would probably make it difficult for a typical duo to breed, Dr. Sundar said.
But in a group of three, the result was better. Each year, one adult of this trio, a female, disappeared while the other two nested and laid eggs. “It wasn’t a throuple,” Dr. Sundar said. Only two of the three animals were mated each season.
But when the resulting chick or chicks were about a month old, or immediately after the nest had failed, the absent female reappeared. If there were chicks, she would help feed them. And working together, the three cranes raised a chick almost every two years.
“Finding new behavior like this in a system where we all thought we were monogamous for a long time is very interesting,” said Sahas Barve, 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?”
In some bird species, such as the Florida scrub and Seychelles squirrels, the adult offspring often stay to form a trio with their parents and help raise their siblings, Dr. Barve said.
But Dr. Sundar believes Sarus crane trios are unlikely to include a large chick, based on other research he has done. However, he noted that the third adult could be related in another way. Sharing some genes with the chick could help explain how this system evolved.
If the third adult is unrelated, however, and is not allowed to mate, what is the benefit of living in a trio?
“The only benefit we could think of for the third bird is that it is being practiced,” Dr. Sundar said. The helper can learn to defend their home and feed the chicks. At least one trio observed by researchers included a very young male.
Scientists also saw that trios were more common in undesirable habitats. Dr. Sundar believes that team building can be an adaptation to bad circumstances.
Team breeding appears throughout the animal kingdom. Species of monkeys, mongooses, spiders, insects, birds and fish participate in cooperative breeding. Humans too. But so far no one was able to send in the perfect solution, which is not strange.
“These are challenging assumptions we have about this family of birds,” said Anne Lacy, senior director of North American programs at the International Crane Foundation.
Mrs. Lacy said she and her colleagues had never seen threesomes among American cranes, but added, “Could it happen when we’re not watching? Absolutely.”
Dr. Sundar plans to use genetics to find out if sarus crane assistants are related. One question he doesn’t plan to ask, though, is whether the helper is ever the real father of a chick. In other words, is the sarus crane really monogamous?
“These birds are preserved by the mythology that they are all the time with each other and that they are faithful,” he said.
Learning that a percentage of cranes move away from their mates, Dr. Sundar said, runs the risk of damaging the relationship between humans and birds. “Why destroy this mythology for a statistic and a scientific article?” He said.