Cranes have a reputation for being romantic. The birds live in faithful pairs, dancing and defending their territory together. When intruders approach, the birds raise their bills and utter a loud single-voiced song.
In India, the sarus crane – crimson-headed and as tall as an adult human – is famous for its monogamy. “When one of the birds dies, the local mythology has it that the other bird is pining for grief,” said KS Gopi Sundar, a scientist at the Nature Conservation Foundation in India. “The truth is, of course, a little different.”
Dr Sundar found that pairs of sarus cranes sometimes let a third bird join them. He described the behavior last month in the journal Ecology. Living in a trio – alas, not quite a group – can help birds raise their young in poor conditions, with one perhaps acting a bit like an avian au pair. The birds even turn their signature duet into a song for three.
Dr Sundar first spotted a trio of sarus cranes in 1999. “When I told experts about it in the US, they smiled and patted me on the head,” he said . But he wasn’t ready to give up on the idea. He followed this trio for the next 16 years.
From 2011, he also trained field assistants (usually local farmers) to monitor sarus cranes. After collecting data up to 2020, Dr. Sundar and Swati Kittur, a colleague at the foundation, dug into this database to search for trios.
Observers had spotted 193 trios among more than 11,500 crane sightings. “So trios are definitely rare,” Dr. Sundar said. Some included one male and two females; some were the other way around.
Suhridam Roy, a graduate student at the foundation, visited four of these trios and played recordings of other crane pairs singing their territorial duets. In response, each trio made their own synchronized call. Scientists called it a triet.
The data does not reveal how many chicks these trios raised or how long they stayed together. But 16 years of observing this quirky trio has given some clues to their family dynamics.
These cranes lived in poor quality habitat, where the lack of wetlands would most likely make it difficult for a typical pair to raise young, 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 – disappeared while the other two nested and laid eggs. “It wasn’t a band,” Dr Sundar said. Only two of the three animals mated each season.
But when the resulting chick(s) were about a month old, or immediately after nest failure, the absent female would reappear. If there were chicks, she helped feed them. And working together, the three cranes raised a chick almost every two years.
“Finding a new behavior like this in a system where we all thought they were monogamous for a long time is super interesting,” said Sahas Barve, an evolutionary ecologist at the National Museum of Natural History at the Smithsonian Institution in Washington, D.C. DC.
And the study raises a lot of questions, he said. The most important: “Who is this third bird?”
In some bird species, including Florida jays and Seychelles warblers, adult offspring often stay behind to form a trio with their parents and help raise their siblings, Dr Barve said.
But Dr Sundar thinks the sarus crane trios are unlikely to include an adult chick, based on other research he has done. However, he noted that the third adult could be related in another way. Sharing certain genes with the chick could help explain how this system evolved.
If the third adult is unrelated, however – and if they are not allowed to mate – what advantage does they derive from living in a trio?
“The only benefit we could think of for the third bird is that it trains,” Dr Sundar said. The helper can learn to defend his home and feed the chicks. At least one trio the researchers observed included a very young male.
The scientists also found that trios were more common in undesirable habitats. Dr. Sundar thinks teaming up can be an adaptation to bad circumstances.
Team parenting appears throughout the animal kingdom. Species of monkeys, mongooses, spiders, insects, birds and fish engage in cooperative breeding. Humans too. But until now, no cranes were known to form teams.
“These are tough assumptions we have about this family of birds,” said Anne Lacy, senior manager of North American programs for the International Crane Foundation.
Ms Lacy said she and her colleagues had never observed trios among North American cranes, but added: “Could this happen when we just aren’t looking? Absolutely.”
Dr. Sundar plans to use genetics to find out if the sarus crane helpers are relatives. One question he doesn’t intend to ask, however, is whether the helper is ever a chick’s true parent. In other words, is the sarus crane really monogamous?
“These birds are preserved for the mythology that they are together all the time and are faithful,” he said.
Learning that a certain percentage of cranes stray from their mates, Dr Sundar said, risks damaging the relationship between man and bird. “Why destroy this mythology for a statistic and for a scientific article?” he said.