JINAN, Dec. 17 -- The number of bird species in the Yellow River Delta nature reserve has nearly doubled thanks to protection efforts in the past two decades, administration authorities said on Tuesday.

The number of bird species has increased to 367 from 187 when the reserve was established in 1992, said Xin Hongquan, an official with the reserve.

The population of birds in the vast wetland has grown from two million to six million with the improvement of its ecological environment, he told Xinhua.

The reserve is now home to 12 bird species on the country's top protection list, up from five upon establishment. Its population of oriental white storks, red-crowned cranes and black-headed gulls, respectively account for 11.7 percent, 15.2 percent and 25.8 percent of these rare species worldwide.

Located at the estuary of the Yellow River in Dongying City, Shandong Province, the wetland reserve covers an area of 153,000 hectares.

Since 2002, its ecological environment has improved a lot with the implementation of wetland restoration projects with an investment of 180 million yuan (295 million U.S. dollars) and the country's regulation of the river's water and sand.

The 5,464-km-long Yellow River, China's second longest, originates in the Qinghai-Tibet Plateau, wriggles its way eastward and finally empties into the Bohai Sea in Shandong.