Wu Xiaomin, an expert from Shaanxi Institute of Zoology, said the population of the once-endangered species has increased dramatically in the last few decades thanks to enhanced protection efforts -including more aggressive enforcement of anti-poaching laws and measures to improve the animals' habitat.

The antelope are a first-class protected species in China, but they are making good progress in numbers. The population of the species has grown from 60,000 or 70,000 in the early 1990s to around 300,000 now.

"Every year starting from around May, all the female antelopes, including 2-year-olds, take part in the migration," said Wu, who also works for the Northwest Institute of Endangered Zoological Species.

Wu said the habitat for the Tibetan antelope in China covers more than a million square kilometers, mostly in remote, severe environments with altitudes between 3,700 meters and 5,500 meters.

"Because of climate change, the migration time of the antelopes during which they give birth is coming a few days earlier, and their longest migratory route could reach to 800 km if measured in a straight line," Wu said.

More than 90 percent of the female animals give birth every year, but only about 30 percent of the newborns survive. Many of the babies die on the journey, unable to hold up under the extreme conditions, which include predators and disease.

Wu said there are more than 200,000 of the antelopes in Tibet's northern Changthang grassland, more than 70,000 in the Hoh Xil areas and more than 20,000 in Xinjiang.

From a survey trip he took to observe the migration in July 2020, he happened to observe a herd of more than 60,000 migrating antelopes at close range in Rongma township of Tibet's Nyima county.

"I was shocked to see such a big herd," he said. "I felt what turned out to be a magnitude 6.6 earthquake early in the morning, but I had a pleasant time for the rest of that day."