Tourists watch and feed the black-headed gulls near the Dianchi Lake in Kunming, southwest China's Yunnan Province, Feb. 21, 2018. Every year in autumn and winter, black-headed gulls flew to Kunming for warmer weather. (Xinhua/Hu Chao)

Tourists feed the black-headed gulls near the Dianchi Lake in Kunming, southwest China's Yunnan Province, Feb. 21, 2018. Every year in autumn and winter, black-headed gulls flew to Kunming for warmer weather. (Xinhua/Hu Chao)

A black-headed gull takes food near the Dianchi Lake in Kunming, southwest China's Yunnan Province, Feb. 21, 2018. Every year in autumn and winter, black-headed gulls flew to Kunming for warmer weather. (Xinhua/Hu Chao)