White swans are seen at a wetland in Sanmenxia, central China's Henan Province, Jan. 31, 2018. Thousands of migratory white swans spend winter at the wetland. (Xinhua/Zhu Xiang)

White swans are seen at a wetland in Sanmenxia, central China's Henan Province, Jan. 31, 2018. Thousands of migratory white swans spend winter at the wetland. (Xinhua/Zhu Xiang)

White swans are seen at a wetland in Sanmenxia, central China's Henan Province, Jan. 31, 2018. Thousands of migratory white swans spend winter at the wetland. (Xinhua/Zhu Xiang)

White swans are seen at a wetland in Sanmenxia, central China's Henan Province, Jan. 31, 2018. Thousands of migratory white swans spend winter at the wetland. (Xinhua/Zhu Xiang)

White swans are seen at a wetland in Sanmenxia, central China's Henan Province, Jan. 31, 2018. Thousands of migratory white swans spend winter at the wetland. (Xinhua/Zhu Xiang)