XINING -- Pregnant Tibetan antelopes have begun their annual migration to the heart of Northwest China's Hoh Xil National Nature Reserve, where they intend to give birth, the reserve's management office said Monday.

This year's annual migration of Tibetan antelopes began nine days earlier than last year. Since April 26, over 270 antelopes have embarked on their journey.

Wu Xiaomin, an expert on the species' protection, attributed the reason behind the advanced annual migration to the steady growth of the antelopes' population, the species' distribution tendency to spread and to get closer to monitoring stations as well as climate changes over the years.

Under the first-class state protection in China, the once-endangered species is mostly found in the country's Tibet autonomous region, Qinghai province, and Xinjiang Uygur autonomous region. The species plays a key role in maintaining the ecological balance on the Qinghai-Tibet Plateau.