Migratory birds are seen at Ninghu wetland park in Xining, northwest China's Qinghai Province, Nov. 18, 2018. The park saw a flock of migratory birds overwintering in it recently. (Xinhua/Wu Gang)

　　Migratory birds are seen at Ninghu wetland park in Xining, northwest China's Qinghai Province, Nov. 18, 2018. The park saw a flock of migratory birds overwintering in it recently. (Xinhua/Wu Gang)

　　Migratory birds are seen at Ninghu wetland park in Xining, northwest China's Qinghai Province, Nov. 18, 2018. The park saw a flock of migratory birds overwintering in it recently. (Xinhua/Wu Gang)