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Swans are seen in the Beidagang Wetland in north China's Tianjin, Dec. 3, 2019. Migratory birds have flocked to the wetland for a winter stopover. (Xinhua/Li Ran)
 
  
Swans are seen in the Beidagang Wetland in north China's Tianjin, Dec. 3, 2019. Migratory birds have flocked to the wetland for a winter stopover. (Xinhua/Li Ran)