Where did the swan come from in Sanjiangkou, Lanzhou?

Siberia. Every year, many white swans migrate from distant Siberia to the Sanjiangkou in Dachuan, Lanzhou for the winter. A swan is shaped like a goose, but its body is very big. It has a white body, a yellow and black upper mouth, short legs and tail, black feet and webbed feet. It is a migratory bird in the constellation Cygnus.