About 18,000 summer birds have been forced to leave their stopover at Honghu Lake early because of the persistent drought in Central China.

Birds fly over the wetland in the East Dongting Lake National Nature Reserve in Yueyang City of central China's Hunan Province, June 2, 2011. [Xinhua]

"Normally, about 20,000 summer birds come to the lake in early April and fly away by the end of August," said Wen Feng, head of marsh protection of the Honghu Wetland Nature Reserve.

"But this year, most of the birds have come and gone. Only about 2,000 to 3,000 birds have stayed," Wen said.

He added that the dried lake has affected spawning and killed water plants, fish and clams - all primary food for birds.?

"Migratory birds are very sensitive to the marsh environment. The fewer birds that stay, the worse the environment is."

"And the local ecology will not recover before more birds come in September to pass the winter."

Local marsh administrators on Thursday told China Daily that the drought - which has turned the lake into a "grassland" - will affect another 40,000 to 80,000 migratory birds that would usually pass the winter here.

A dozen types of migratory birds en route from East Asia to Australasia temporarily inhabit the lake, according to Wen.

The lake area is also home to 494 kinds of plants and 774 kinds of animals.

Rainfall along the middle and lower reaches of the Yangtze River has been at its lowest since 1951, down 40 percent to 60 percent from its average level, according to the Office of State Flood Control and Drought Relief Headquarters.

Honghu Lake - a key reserve of the river's water in the middle reaches and dubbed "land of fish and rice" - has shrunk from about 35,000 hectares to 4,500 hectares.

Turbid waters run shallow on the edge of the lake land that cracks and grows fresh grass, while in the center of the lake the water is a mere 30 cm deep. The lakeside smells of dead fish and clams.

Wang Limin, deputy conservation director of the World Wide Fund for Nature China, said a current assessment being done by the group to assess the drought's influence on four sections of marsh in the region is worrisome.

The marsh ecological systems of Honghu and Tian'ezhou in Hubei province, Dongting Lake in Hunan province, and Chenhang Reservoir in Shanghai are "severely damaged" and species living in and around are "threatened", Wang said, citing the consensus of a group of 10 experts in natural resources and ecology who just finished on-site surveys in the regions.

"The experts agreed the drought is a collective outcome of climate change and hydro projects," Wang said. "The size of the marshes has dwindled, structures damaged and functions degraded."

Wang added experts all believed a management and early-alarm system should be established to address such "extreme cases".