"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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