Thirteen oriental white storks died from pesticide poisoning in North China’s Tianjin on Tuesday, but another 13 poisoned white storks have recovered well, an expert involved in the rescue said.

“The official laboratory report will be released by the municipal government in a week, but based on what we found during the rescue, it’s probably the pesticide containing organophosphates, which can attack the nervous system, that killed the 13 white storks,” said Rong Wancheng, director of the Agricultural Service Center in Dagang, Tianjin, which cooperated with a local breeding center to treat the injured birds.

Oriental white storks are listed as rare birds under first-grade State protection in China. There are only 2,500 of the species in the world, according to research by Bird Life International, a conservation group headquartered in Cambridge, UK.

The birds rest in Tianjin and Hebei province during their long flight south for the winter. In Tianjin, the Beidagang Natural Reserve is the major habitat for them.

The poisoned birds were discovered by a wildlife photographer on Sunday, who raised the alarm that the birds were unwell.

“Two of them are in a serious condition but are not critical. All of them may continue their migration after their recovery,” Rong said.

Other birds, including wild ducks, have also been killed. The bodies of the rare white storks will be given to the local natural museum. Many volunteers and officials from the government have been working in the reserve to save more poisoned birds.

Rong said there was a tradition of hunting before the 1980s in the area, and these white storks can be eaten in the hours after being poisoned.

The local police bureau started investigating the matter on Sunday.