The oriental white storks should have left for the south by the end of November but are still living on the Beidagang Wetland Nature Reserve, which has received a great deal of ice and snow, said Wang Jianmin, a volunteer at the Tianjin Wild Animal Rescue and Training Center.

Heavy snow hit the city on Friday, making it difficult for the birds to find food, as the wetland has largely frozen.

Wang said the birds may have been poisoned by tainted food or simply became too weak to fly away.

Volunteers from the center spread small fish on the ice Friday night, all of which was consumed by the storks by the next morning. Food was put out for the birds again Saturday morning and the center plans to offer more food on Sunday.

"If they still don't fly south after eating, it will be a serious problem," said Wang, adding that the case may be related to a recent poisoning incident.

In November, 20 oriental white storks were found dead and another 13 poisoned in the wetland after the waters were confirmed to be tainted with pesticide. Two suspects, who were found to have dispersed the pesticide for poaching purposes, were caught by police on Nov. 25.

The oriental white stork is included in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which means the species is one of the most endangered among CITES-listed animals and plants. Only 2,500 to 3,000 of the birds exist.

The species, which usually reproduces in the northeast, migrates south for winter. The Beidagang Wetland is an important habitat located along their migratory route.