Swans are seen in the Yellow River wetland in Guide County, northwest China's Qinghai Province, Dec. 12, 2019. Hundreds of migratory swans flied to Guide for wintering. (Xinhua/Zhang Long)

  Swans are seen in the Yellow River wetland in Guide County, northwest China's Qinghai Province, Dec. 12, 2019. Hundreds of migratory swans flied to Guide for wintering. (Xinhua/Zhang Long)

  Swans are seen in the Yellow River wetland in Guide County, northwest China's Qinghai Province, Dec. 12, 2019. Hundreds of migratory swans flied to Guide for wintering. (Xinhua/Zhang Long)

  Swans are seen in the Yellow River wetland in Guide County, northwest China's Qinghai Province, Dec. 12, 2019. Hundreds of migratory swans flied to Guide for wintering. (Xinhua/Zhang Long)

  Swans are seen in the Yellow River wetland in Guide County, northwest China's Qinghai Province, Dec. 12, 2019. Hundreds of migratory swans flied to Guide for wintering. (Xinhua/Zhang Long)

  Swans are seen in the Yellow River wetland in Guide County, northwest China's Qinghai Province, Dec. 12, 2019. Hundreds of migratory swans flied to Guide for wintering. (Xinhua/Zhang Long)

  Swans are seen in the Yellow River wetland in Guide County, northwest China's Qinghai Province, Dec. 12, 2019. Hundreds of migratory swans flied to Guide for wintering. (Xinhua/Zhang Long)

  Swans are seen in the Yellow River wetland in Guide County, northwest China's Qinghai Province, Dec. 12, 2019. Hundreds of migratory swans flied to Guide for wintering. (Xinhua/Zhang Long)