A?pair of black swans returned to the Huangpu River yesterday, after first being sighted at the weekend - but where they came from remains a mystery.

The pair of black swans swim on the Huangpu River near the Bund yesterday. [Shanghai Daily]

As there is little food for the birds in the river, Shanghai Wildlife Conservation Center said it will try to catch them and put them in a zoo.

Yesterday morning, the swans were spotted swimming near Binjiang Avenue in the Pudong New Area. A witness, surnamed Zhang, said he rarely saw any birds on the Huangpu River, certainly not black swans.

When the swans appeared for the first time on Saturday, a center patrol boat tried to catch them. "It's difficult to get them on the water," said Pei Enle, director of the center.

Pei said the swans, which are native to Australia, would struggle to find enough food. He reckoned they would go to the riverbank when they are very hungry, and that officials could catch them then.

The center said the swans may have come from a park or farm.

"In Shanghai, some farmers raise black swans as ornamental birds or as food," said Pei.