Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year. (Photos: Xinhua)

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.

Photo taken on Feb. 21, 2019 shows black-necked cranes at Caohai National Nature Reserve in Weining County, southwest China's Guizhou Province. Caohai National Nature Reserve is one of the main habitats for black-necked cranes to winter from late October to the end of March of next year.