Oriental white storks, a China's national first-class protected animal, fly over the Beidagang Wetland in north China's Tianjin, Dec. 3, 2019. Migratory birds have flocked to the wetland for a winter stopover. (Photo: Xinhua)

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.

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.

Oriental white storks, a China's national first-class protected animal, fly over the Beidagang Wetland in north China's Tianjin, Dec. 3, 2019. Migratory birds have flocked to the wetland for a winter stopover.

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.

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.

Migrant birds fly over the Beidagang Wetland in north China's Tianjin, Dec. 3, 2019. Migratory birds have flocked to the wetland for a winter stopover.