Photo taken on Dec. 3, 2019 shows migrant birds resting at Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Xinhua/Chen Zhenhai)

  Photo taken on Dec. 2, 2019 shows two migrant birds at Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Photo by Guo Yudi/Xinhua)

  Photo taken on Dec. 3, 2019 shows migrant birds resting at Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Xinhua/Chen Zhenhai)

  Photo taken on Dec. 2, 2019 shows two migrant birds at Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Xinhua/Chen Zhenhai)

  Photo taken on Dec. 2, 2019 shows migrant birds flying above Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Xinhua/Chen Zhenhai)

  Photo taken on Dec. 2, 2019 shows migrant birds flying at Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Photo by Guo Yudi/Xinhua)

  Photo taken on Dec. 2, 2019 shows migrant birds flying above Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Xinhua/Chen Zhenhai)

  Photo taken on Dec. 2, 2019 shows migrant birds flying above Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Photo by Guo Yudi/Xinhua)

  Photo taken on Dec. 2, 2019 shows migrant birds flying above Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Xinhua/Chen Zhenhai)

  Photo taken on Dec. 2, 2019 shows a migrant bird at Hengling Lake Provincial Nature Reserve, a part of Dongting Lake wetland, in Xiangyin County, central China's Hunan province. More than 50,000 migrant birds have recently arrived for spending the winter at Hengling Lake area, according to monitoring data. (Xinhua/Chen Zhenhai)