Swans forage in Luoping Lake, a stop-over for wintering migratory birds, in the Chabei area of Zhangjiakou, Hebei, Nov. 18, 2018. (Photos: Xinhua)

Swans forage in Luoping Lake, a stop-over for wintering migratory birds, in the Chabei area of Zhangjiakou, north China's Hebei Province, Nov. 18, 2018.

Swans forage in Luoping Lake, a stop-over for wintering migratory birds, in the Chabei area of Zhangjiakou, north China's Hebei Province, Nov. 18, 2018.

Swans frolic in Luoping Lake, a stop-over for wintering migratory birds, in the Chabei area of Zhangjiakou, north China's Hebei Province, Nov. 18, 2018.