Black-necked cranes gather in Zoige Wetland National Nature Reserve in Sichuan province before starting their autumn migration south. TAN XI/FOR CHINA DAILY

The black-necked crane, a symbol of fidelity on the Qinghai-Tibet Plateau, and a species historically perceived as loyal, are often seen in pairs in wetlands or highland barley fields.

However, a recent groundbreaking study has shattered this long-held belief, unveiling that the traditional notion surrounding these majestic birds is nothing more than a human illusion.

The study, conducted by a team of Chinese researchers, sheds light on the mysterious mating strategies and group dynamics of the black-necked crane (Grus nigricollis), a species mainly found on the Qinghai-Tibet Plateau.

Utilizing advanced DNA analysis techniques and meticulous field observations, the study challenges conventional views of crane monogamy and unveils intriguing insights into the species' reproductive behavior.

The research, spanning four years and focusing on breeding pairs in important bird habitats such as Flower Marsh and Nalecho Marsh in Zoige Wetland National Nature Reserve in Sichuan province, revealed a complex tapestry of mating strategies among this wetland species.

Contrary to expectations of strict monogamy, the study found a significant tendency for extra-pair copulation, with no pair maintaining monogamy throughout the observation period. Notably, the extra-pair fertilization rate was found to be 16.6 percent, a figure higher than in many other territorial bird species.

The research, titled "Mechanisms of enhancing genetic fitness in non-kinship-based groups: insights from black-necked cranes", was published online on Thursday on the open access, peer-reviewed science journal, The Ecological Indicator.

"Our findings are kind of contrary to our traditional understanding," said Li Yuhang, a researcher from Sichuan University in Chengdu, Sichuan province, who is one of the article's two first authors. "The crane is considered a symbol of fidelity, not only in China but also in Bhutan and India."

An egg gestates in a black-necked crane nest at Flower Marsh. CHINA DAILY

The black-necked crane mainly summers in the Qinghai-Tibet Plateau. The breeding areas are alpine meadows, lakesides and riverine marshes and river valleys. Their wintering areas extend to parts of Yunnan and Guizhou province in China, Bhutan and the northern plains of India. According to the article, the crane is recognized as a unique representative of high-altitude cranes. It has maintained its status as one of the least-studied cranes.

Li and his research partners from Sichuan University, Nanjing University and Zoige nature reserve focused their study on cranes during breeding season, when the birds establish sparsely spaced colonies, suggesting the potential for high mate fidelity due to low breeding density, according to the article.

From 2017 to 2020, the researchers conducted annual fieldwork in Flower Marsh from March to October, rigorously monitoring the cranes. "The cranes we studied in Zoige are considered to be part of the bird's eastern population," Li told China Daily. "They usually winter in Guizhou and Yunnan provinces."

They identified and tagged each breeding pair, located their nests and recorded satellite navigation coordinates. Over these four breeding seasons, they documented nine nests in Flower Marsh. In addition to the nest-area samples from Flower Marsh, they expanded their dataset to include tissue samples, egg membrane samples and nine blood samples from both Nalecho Marsh and Flower Marsh.

In total, 48 egg membrane samples from offspring, nine blood samples, four tissue samples and 65 adult feather samples were collected in Zoige for DNA extraction.

The researchers identified 58 individual birds over the four-year period — seven female and eight male adults, and 25 male and 18 female offspring.

"Black-necked cranes are large birds under State first-class protection," said Professor Li Zhongqiu from Nanjing University's School of Life Science in Jiangsu province, who is one of the article's co-authors. "In the past, if we wanted to study their tendency for extra-pair copulation, we had to try to catch the breeding pairs and put satellite tracking devices on them, and wait to see whether the same pairs would return to the same nest the next year. The method is actually hard to carry out and is unreliable."

Documenting nine nests and collecting samples for analysis posed a different challenge, the professor said.

Jin Yuyi, a researcher from Sichuan University, collects data and samples from a black-necked crane nest in Zoige. CHINA DAILY

"Each nest was far from the others. Our colleagues had to drive a long distance to monitor the birds in the nests," Li Zhongqiu said. "Also, they must be very careful to collect the samples so as not to disturb the breeding birds."

Their findings indicated a 100 percent probability that at least one parent from the nests they studied returned in subsequent seasons, demonstrating high nest-site fidelity among the cranes in their study area.

"Based on our defined criteria for nest-site fidelity, we considered an individual exhibiting fidelity if it utilized the same nest at least twice during the observation period," Li Yuhang said.

Overall, the article revealed that nest-site fidelity was 100 percent for males and 71 percent for females.

The migration of the black-necked crane from wintering to breeding grounds, covering distances from about 200 kilometers to as much as 1,500 km, underscores the remarkable feat of their nest-site fidelity.

"As we know, this is the first report describing nest-site fidelity in this elusive species," Li Yuhang said, adding that high fidelity to a nesting area offers several benefits, including familiarity with foraging and shelter sites, mating partners and neighbors.

Among the seven breeding pairs documented with at least two breeding instances, none maintained fidelity to each other throughout the entire observation period. "In simple words, no pairing endured throughout four years," Li Yuhang said.

In cases where two eggs were identified in the same nest during the same breeding season, the proportion of extra-pair fertilization eggs was 16.6 percent.

This rate is notably higher than in many other territorial bird species, according to the article.

The significant incidence of extra-pair fertilization may have been a critical factor in the rapid recovery of the black-necked crane population, which grew from an estimated 100 to 300 individuals at the end of the 20th century to nearly 15,000 by 2020.

"Such behaviors likely facilitated enhanced gene flow and maintained genetic diversity within the species' confined breeding ranges, contributing to the resilience and growth of the population," Li Yuhang said, adding that studies have shown that climate change can also influence the proportion of extra-pair breeding, where such mating behaviors increase offspring genetic diversity, enhancing population resilience to environmental changes.

Despite the significant findings, the researchers also said that their findings must be interpreted cautiously due to the relatively small sample size. Future research with long-term observations to obtain larger sample sizes, combined with more robust statistical methods, is necessary to confirm these findings and provide more definitive conclusions.

"It will help refine conservation strategies and improve our understanding of the ecological dynamics affecting black-necked cranes in the face of environmental changes," said professor Li Zhongqiu.

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