Police in northern China are investigating the suspected poisoning of 233 wild swans. The birds were found dead along with 26 mallard ducks on Hongtu Lake in Inner Mongolia, an autonomous Chinese region.

Officials suspect poachers are responsible, and a reward of 100,000 yuan (about $15,000) has been offered to anyone with information that leads to an arrest.  The bird deaths were reported by an elderly couple visiting Hongtu Lake (or "Lake of the Swans") to see the animals' annual migration.

Though tundra swans (Cygnus columbianus) are widespread across the globe, those that pass through Inner Mongolia are classed as a subspecies colloquially known as the Bewick's swan (Cygnus columbianus bewickii). Some 10,000 of these elegant birds breed in eastern Russia, and 2,000 of them have been known to settle on Hongtu Lake on their way to their southern wintering grounds. In recent years, however, that number has fallen to 1,400.

Exactly why the dead swans were left in the lake is unclear at this point, but officials say the animals did not die of natural causes. Traces of carbofuran, a toxic pesticide commonly used by poachers, were found in the tissues. Back in April, forestry police discovered 200 birds had been killed by corn kernels laced with this same deadly substance. 

According to Tian YangYang, who works with environmental organisation Let Birds Fly, the animals were not destined for use in traditional medicine, unlike many other poached species. "Most of the dead birds end up at restaurants," he told Sixth Tone. Even though such birds are killed with poison, he notes, patrons who participate in so-called "swan feasts" believe eating the meat is safe.

The State Forestry Administration (SFA) will continue investigating the case over the coming weeks, and officials are urging anyone who witnesses suspicious activity to come forward.

The deaths come just days after the administration announced a new initiative to "strengthen the focus on migratory birds" by clearing poaching nets from known hotspots. Six enforcement groups are to be dispatched along avian migration routes to case for illegal activity. "We have a responsibility to set goals, and clamp down on illegal black-market trading," SFA said in an official statement [translated from Chinese].

Some experts are concerned that without action, birds will begin to abandon the region's waters for safer locales, which could have lasting effects on the local ecosystem.
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Top header image: Michael Day/Flickr