Brown-headed gulls are seen along the bank of Nam Co Lake in southwest China's Tibet Autonomous Region, July 5, 2019. (Xinhua/Li Xin)

A tourist takes pictures of a brown-headed gull along the bank of Nam Co Lake in southwest China's Tibet Autonomous Region, July 5, 2019. (Xinhua/Zhou Jinshuai)

Brown-headed gulls are seen along the bank of Nam Co Lake in southwest China's Tibet Autonomous Region, July 5, 2019. (Xinhua/Zhou Jinshuai)

Brown-headed gulls are seen along the bank of Nam Co Lake in southwest China's Tibet Autonomous Region, July 5, 2019. (Xinhua/Zhou Jinshuai)

Brown-headed gulls are seen along the bank of Nam Co Lake in southwest China's Tibet Autonomous Region, July 5, 2019. (Xinhua/Li Xin)

Brown-headed gulls are seen on Nam Co Lake in southwest China's Tibet Autonomous Region, July 5, 2019. (Xinhua/Zhou Jinshuai)

A brown-headed gull flies over Nam Co Lake in southwest China's Tibet Autonomous Region, July 5, 2019. (Xinhua/Li Xin)

Brown-headed gulls are seen along the bank of Nam Co Lake in southwest China's Tibet Autonomous Region, July 5, 2019. (Xinhua/Zhou Jinshuai)

Brown-headed gulls are seen on Nam Co Lake in southwest China's Tibet Autonomous Region, July 5, 2019. (Xinhua/Zhou Jinshuai)