People feed black-headed gulls by the Yange Lake in Yinchuan, capital of northwest China's Ningxia Hui Autonomous Region, March 21, 2019. (Xinhua/Feng Kaihua)

People feed black-headed gulls by the Yange Lake in Yinchuan, capital of northwest China's Ningxia Hui Autonomous Region, March 21, 2019. (Xinhua/Feng Kaihua)

Black-headed gulls fly over the Yange Lake in Yinchuan, capital of northwest China's Ningxia Hui Autonomous Region, March 21, 2019. (Xinhua/Feng Kaihua)

People feed black-headed gulls by the Yange Lake in Yinchuan, capital of northwest China's Ningxia Hui Autonomous Region, March 21, 2019. (Xinhua/Feng Kaihua)

Black-headed gulls scramble for food on the Yange Lake in Yinchuan, capital of northwest China's Ningxia Hui Autonomous Region, March 21, 2019. (Xinhua/Feng Kaihua)

A woman takes photo of black-headed gulls by the Yange Lake in Yinchuan, capital of northwest China's Ningxia Hui Autonomous Region, March 21, 2019. (Xinhua/Feng Kaihua)

Black-headed gulls are seen on the Yange Lake in Yinchuan, capital of northwest China's Ningxia Hui Autonomous Region, March 21, 2019. (Xinhua/Feng Kaihua)