A black-headed gull flies over the Haihe River in north China's Tianjin, Feb. 27, 2021. In recent years, with the improvement of the ecological environment in Tianjin, the water quality of Haihe River has been continuously ameliorated, attracting many black-headed gulls from late November to March the next year. (Photo by Sun Fanyue/Xinhua)

A black-headed gull flies over the Haihe River in north China's Tianjin, Feb. 27, 2021. In recent years, with the improvement of the ecological environment in Tianjin, the water quality of Haihe River has been continuously ameliorated, attracting many black-headed gulls from late November to March the next year. (Photo by Sun Fanyue/Xinhua)

A black-headed gull forages at the Haihe River in north China's Tianjin, Feb. 27, 2021. In recent years, with the improvement of the ecological environment in Tianjin, the water quality of Haihe River has been continuously ameliorated, attracting many black-headed gulls from late November to March the next year. (Photo by Sun Fanyue/Xinhua)

A black-headed gull flies over the Haihe River in north China's Tianjin, Feb. 27, 2021. In recent years, with the improvement of the ecological environment in Tianjin, the water quality of Haihe River has been continuously ameliorated, attracting many black-headed gulls from late November to March the next year. (Photo by Sun Fanyue/Xinhua)

A black-headed gull forages at the Haihe River in north China's Tianjin, Feb. 27, 2021. In recent years, with the improvement of the ecological environment in Tianjin, the water quality of Haihe River has been continuously ameliorated, attracting many black-headed gulls from late November to March the next year. (Photo by Sun Fanyue/Xinhua)

来源：Xinhua