Mallards swim in the Lhasa River, southwest China's Tibet Autonomous Region, Dec. 24, 2017. The populations of bar-headed goose, ruddy shelduck and mallard have increased in Lhasa River. (Xinhua/Zhang Rufeng)

Bar-headed geese fly over the Lhasa River, southwest China's Tibet Autonomous Region, Dec. 24, 2017. The populations of bar-headed goose, ruddy shelduck and mallard have increased in Lhasa River. (Xinhua/Zhang Rufeng)

Bar-headed geese fly over the Lhasa River, southwest China's Tibet Autonomous Region, Dec. 24, 2017. The populations of bar-headed goose, ruddy shelduck and mallard have increased in Lhasa River. (Xinhua/Zhang Rufeng)

Birds swim in the Lhasa River, southwest China's Tibet Autonomous Region, Dec. 24, 2017. The populations of bar-headed goose, ruddy shelduck and mallard have increased in Lhasa River. (Xinhua/Zhang Rufeng)

Bar-headed geese fly over the Lhasa River, southwest China's Tibet Autonomous Region, Dec. 24, 2017. The populations of bar-headed goose, ruddy shelduck and mallard have increased in Lhasa River. (Xinhua/Zhang Rufeng)

Ruddy shelducks fly over the Lhasa River, southwest China's Tibet Autonomous Region, Dec. 24, 2017. The populations of bar-headed goose, ruddy shelduck and mallard have increased in Lhasa River. (Xinhua/Zhang Rufeng)

A ruddy shelduck swims in the Lhasa River while another one flies over the river, southwest China's Tibet Autonomous Region, Dec. 24, 2017. The populations of bar-headed goose, ruddy shelduck and mallard have increased in Lhasa River. (Xinhua/Zhang Rufeng)