Migratory birds rest at a wetland in Qinhuangdao, north China's Hebei Province, April 4, 2019. (Xinhua/Yang Shiyao)

Migratory birds fly over a wetland in Qinhuangdao, north China's Hebei Province, April 4, 2019. (Xinhua/Yang Shiyao)

Migratory birds rest at a wetland in Qinhuangdao, north China's Hebei Province, April 4, 2019. (Xinhua/Yang Shiyao)

Migratory birds fly over a wetland in Qinhuangdao, north China's Hebei Province, April 4, 2019. (Xinhua/Yang Shiyao)