File photo taken on March 9, 2018 shows a flock of black-necked cranes at the Caohai National Nature Reserve in the Yi, Hui and Miao Autonomous County of Weining, southwest China's Guizhou Province. There was a time when the Caohai National Nature Reserve was threatened by water body recession and pollution, but in recent years its ecological system has greatly improved after a series of control measures were implemented here. (Xinhua/Yang Wenbin)
