White swans are seen at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 22, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans.

White swans are seen at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 20, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans.

Staff members spread food for the white swans at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 20, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans.

Visitors look at white swans at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 17, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans.

A staff member spreads food for the white swans at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 20, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans.

White swans are seen at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 22, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans.

White swans are seen at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 22, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans. (Photos: Xinhua/Zhu Xiang)

A white swan wearing a track collar is seen at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 22, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans.

White swans are seen at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 17, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans.

A girl plays at the Yellow River wetland in Sanmenxia, central China's Henan Province, Nov. 17, 2018. White swans have flown from Siberia to the Yellow River wetland to spend winter. Measures have been taken to protect these migratory swans.