Photo taken on April 4, 2020 shows migrant birds flying over the Ulansuhai Nur wetland in Bayannur, north China's Inner Mongolia Autonomous Region. (Xinhua/Liu Lei)

Photo taken on April 4, 2020 shows a migrant bird flying over the Ulansuhai Nur wetland in Bayannur, north China's Inner Mongolia Autonomous Region. (Xinhua/Lian Zhen)

Photo taken on April 4, 2020 shows a migrant bird in the Ulansuhai Nur wetland in Bayannur, north China's Inner Mongolia Autonomous Region. (Xinhua/Lian Zhen)

Photo taken on April 4, 2020 shows migrant birds in the Ulansuhai Nur wetland in Bayannur, north China's Inner Mongolia Autonomous Region. (Xinhua/Lian Zhen)

Photo taken on April 4, 2020 shows migrant birds in the Ulansuhai Nur wetland in Bayannur, north China's Inner Mongolia Autonomous Region. (Xinhua/Lian Zhen)

Photo taken on April 4, 2020 shows a migrant bird resting in the Ulansuhai Nur wetland in Bayannur, north China's Inner Mongolia Autonomous Region. (Xinhua/Lian Zhen)

Photo taken on April 4, 2020 shows a migrant bird in the Ulansuhai Nur wetland in Bayannur, north China's Inner Mongolia Autonomous Region. (Xinhua/Lian Zhen)

Photo taken on April 4, 2020 shows a migrant bird flying over the Ulansuhai Nur wetland in Bayannur, north China's Inner Mongolia Autonomous Region. (Xinhua/Lian Zhen)

Photo taken on April 4, 2020 shows migrant birds flying over the Ulansuhai Nur wetland in Bayannur, north China's Inner Mongolia Autonomous Region. (Xinhua/Liu Lei)