Swans are seen in a lake in Yandunjiao Village of Rongcheng City, east China's Shandong Province, Dec. 8, 2015. Rongcheng nature reserve has become a nest for thousands of migratory swans to spend winter. (Xinhua/Wang Haibin) 

Swans are seen in a lake in Yandunjiao Village of Rongcheng City, east China's Shandong Province, Dec. 8, 2015. Rongcheng nature reserve has become a nest for thousands of migratory swans to spend winter. (Xinhua/Wang Haibin)

Swans are seen in a lake in Yandunjiao Village of Rongcheng City, east China's Shandong Province, Dec. 8, 2015. Rongcheng nature reserve has become a nest for thousands of migratory swans to spend winter. (Xinhua/Wang Haibin)

Swans are seen in a lake in Yandunjiao Village of Rongcheng City, east China's Shandong Province, Dec. 8, 2015. Rongcheng nature reserve has become a nest for thousands of migratory swans to spend winter. (Xinhua/Wang Haibin)

Swans fly over a lake in Yandunjiao Village of Rongcheng City, east China's Shandong Province, Dec. 8, 2015. Rongcheng nature reserve has become a nest for thousands of migratory swans to spend winter. (Xinhua/Wang Haibin)

Swans are seen in a lake in Yandunjiao Village of Rongcheng City, east China's Shandong Province, Dec. 8, 2015. Rongcheng nature reserve has become a nest for thousands of migratory swans to spend winter. (Xinhua/Wang Haibin)