For centuries, black-necked cranes have symbolized fidelity and lifelong partnerships in Chinese, Indian, and Bhutanese cultures. But new genetic research has upended that belief, revealing that these revered birds frequently change partners. 

Researchers suggest that this unexpected behavior may be a way to boost genetic diversity and population resilience amid environmental shifts, such as changes to breeding grounds due to climate change.

The groundbreaking study, published in the October issue of the journal Ecological Indicators, comes seven years after the research team — comprising scientists from Sichuan University, Nanjing University, and the Zoigê Wetland National Nature Reserve Administration in Sichuan province — began initial observations in 2017.

Their work included paternity tests using DNA samples from crane parents and chicks found in China. Contrary to expectations of strict monogamy, in which each crane has only one partner at a time, the study found that none of the pairs remained “exclusive” during the four-year observation period.

This marks the world’s first genetic evidence of infidelity among this enigmatic species.

“This discovery has completely shattered our traditional understanding,” Li Zhongqiu, a professor at Nanjing University’s School of Life Sciences and a member of the research team, told Sixth Tone.

Black-necked cranes are the world’s only crane species adapted to high-altitude plateaus. They mostly live on China’s Qinghai-Tibetan Plateau, with smaller groups in Vietnam, Bhutan, and India. Long revered in Buddhist traditions, they symbolize longevity, good fortune, and unwavering fidelity.

In China, folklore depicts black-necked crane couples as inseparable lifelong partners; if one mate dies, the other is said to remain alone. Bhutan honors the bird with an annual festival, while in northern India’s Union Territory of Ladakh, it is celebrated as the state bird.

For years, scientists believed that black-necked cranes adhered more strictly to monogamy than most other bird species. According to Li, while roughly 90% of bird species practice monogamy to some extent, instances of “cheating” are far from rare.

Yet, the behavior of black-necked cranes, with their deeply bonded partnerships, seemed to stand apart — until now.

According to Li, black-necked cranes possess three unique traits. First, they live long lives — often over 50 years — prioritizing quality offspring over quantity. By forming long-term pair bonds, they reduce the likelihood of fertilization from outside partners, which helps ensure a stable genetic mix for their young.

This also reduces competition between partners, enabling them to channel more energy into nurturing their chicks and enhancing reproductive success.

Second, both crane parents share equal responsibility in rearing their young, which reduces the likelihood of infidelity. And finally, they exhibit a strong attachment to their mutual “home,” demonstrating a high level of nest-site fidelity.

Behavioral observations reveal that, unlike many bird species that separate after the breeding season, black-necked crane pairs continue caring for their chicks until the young take their first flight, migrating south as a family for winter.

Jin Yuyi, a researcher and co-author of the study from Sichuan University, described one moving scene they witnessed during their research: a pair of cranes delayed migration to remain with their late-hatched chick, still unable to fly. “Temperatures dropped below minus 10 degrees Celsius. Ice collected on their tail feathers, and they stood on one leg to conserve warmth,” said Jin, who was also responsible for data collection and observation. 

Leaning on each other, the parents endured the harsh winter together. “This led us to believe black-necked cranes wouldn’t easily switch mates,” said Li.

To confirm their hypothesis, the research team braved years of challenging conditions at the Zoigê Wetland Nature Reserve.

Sudden hailstorms blanketed the grasslands with snow, even during summer, Jin recalled. The cranes’ nesting sites — isolated mud islands within vast wetlands — forced team members to wade through water and endure hourslong treks to reach observation points. Patiently waiting for the crane parents to leave their nests, the researchers seized rare moments to collect crucial feather and eggshell samples without disturbing the birds.

Secret lives

When the research team began analyzing feather and eggshell samples following a four-year observation period, they expected to confirm, not challenge, the belief in the species’ loyalty, explained Li Yuhang, the study’s lead author from Sichuan University.

However, none of the crane pairs maintained exclusive partnerships throughout the four breeding seasons. The rate of fertilization from outside partners reached 16.6%, surpassing that of many other territorial bird species, including black swans, black vultures, oystercatchers, and jackdaws.

The findings reveal that black-necked cranes exhibit unexpectedly complex mating behaviors, with no significant differences between males and females in terms of extra-pair activity.

While some partnerships extended beyond a single breeding season, others were limited to just one. Certain cranes engaged with multiple partners within the same season — for example, of the many nests observed, the female from Nest 9 mated with two different males in 2018.

Others changed mates across different seasons, like the male from Nest 3, who fathered chicks with different females in 2017 and 2019. Remarkably, some pairs even reunited after separating, such as the male from Nest 2, who paired again with a former partner in 2018 and 2020, after a different pairing in 2019.

Despite instances of infidelity, black-necked cranes did exhibit strong nest-site fidelity, often reusing the same nest across different breeding seasons. This pattern was noted in nests with offspring recorded during at least two of the four observed breeding seasons, specifically in nests 1, 2, 3, 4, 5, and 9.

Research also shows that male cranes display a higher degree of loyalty to previous nesting sites compared to females, mirroring trends seen in other bird species. While both sexes play a significant role in caring for their young, males concentrate on repairing and defending the nest, while females prioritize energy intake to prepare for reproduction.

This division of roles deepens the attachment of male cranes to their nesting sites, the researchers suggest.

The strong nest-site fidelity and complex mating behaviors of black-necked cranes may represent adaptive responses to shifting environmental conditions. For alpine species like these cranes, climate change could disrupt traditional breeding grounds, affecting their reproductive strategies and survival, according to a 2022 study referenced in the research.

Extra-pair mating behaviors can boost genetic diversity among offspring compared to strict monogamy, potentially enhancing the species’ resilience to environmental changes.

“The significant incidence of extra-pair fertilization may have been a critical factor in the rapid recovery of the black-necked Crane population,” the study notes. From the late 20th century to 2020, the population of the species in China grew from an estimated 100–300 individuals to approximately 15,000.

Returning to the same nesting sites also gives black-necked cranes familiar neighbors, foraging areas, and shelter. To reduce risks, they often travel up to 1,500 kilometers from wintering grounds to proven breeding locations. However, this strong attachment can also pose risks; as habitats change, once-safe nesting sites may become dangerous, leaving the cranes vulnerable.

According to Li Yuhang, these reproductive behaviors are key indicators of ecosystem health and environmental changes. “Humans often observe the environment broadly,” Li said.

“But if black-necked cranes continue returning to traditional nesting sites while facing declining reproductive success, it could signal environmental degradation in that area. This perspective can inform future conservation efforts and policies.”

Editor: Apurva.

(Header image: Black-necked cranes in Lhasa, Xizang Autonomous Region, June 2024. Li Lin/CNS/VCG)