Swans are seen at a wetland in Sanmenxia City, central China's Henan Province, Feb. 2, 2018. Thousands of migratory swans flied here to spend winter at Sanmenxia Yellow River wetland.(Xinhua/Sun Zhiping)

Swans are seen at a wetland in Sanmenxia City, central China's Henan Province, Feb. 2, 2018. Thousands of migratory swans flied here to spend winter at Sanmenxia Yellow River wetland.(Xinhua/Sun Zhiping)

Swans are seen at a wetland in Sanmenxia City, central China's Henan Province, Feb. 2, 2018. Thousands of migratory swans flied here to spend winter at Sanmenxia Yellow River wetland.(Xinhua/Sun Zhiping)

Swans are seen at a wetland in Sanmenxia City, central China's Henan Province, Feb. 2, 2018. Thousands of migratory swans flied here to spend winter at Sanmenxia Yellow River wetland.(Xinhua/Sun Zhiping)