Photo taken on April 5, 2020 shows migrant birds flying over the Yuncheng salt lake wetland in Yuncheng, north China's Shanxi Province. (Xinhua/Wang Feihang)

Photo taken on April 5, 2020 shows migrant birds flying over the Yuncheng salt lake wetland in Yuncheng, north China's Shanxi Province. (Xinhua/Wang Feihang)

Photo taken on April 5, 2020 shows migrant birds flying over the Yuncheng salt lake wetland in Yuncheng, north China's Shanxi Province. (Xinhua/Wang Feihang)

Aerial photo taken on April 5, 2020 shows a view of the salt lake in Yuncheng City, north China's Shanxi Province. (Xinhua/Wang Feihang)

Aerial photo taken on April 5, 2020 shows a view of the salt lake in Yuncheng City, north China's Shanxi Province. (Xinhua/Wang Feihang)

Aerial photo taken on April 5, 2020 shows a view of the salt lake in Yuncheng City, north China's Shanxi Province. (Xinhua/Wang Feihang)

Aerial photo taken on April 5, 2020 shows a view of the salt lake in Yuncheng City, north China's Shanxi Province. (Xinhua/Wang Feihang)

Aerial photo taken on April 5, 2020 shows a view of the salt lake in Yuncheng City, north China's Shanxi Province. (Xinhua/Wang Feihang)

Photo taken on April 5, 2020 shows migrant birds flying over the Yuncheng salt lake wetland in Yuncheng, north China's Shanxi Province. (Xinhua/Wang Feihang)