HOHHOT, Oct. 25  -- Authorities in north China's Inner Mongolia Autonomous Region are investigating the death of 233 swans and have also taken measure to improve the region's protection of migratory birds.

The swans, along with 26 dead mallards, were found in a lake in Xilingol League, according to the local information office.

Photos of the dead birds, which had been removed from the water by herdsmen, were posted across social networks on Sunday.

Post-mortem examinations of the birds and tests of samples of the water in the lake will be carried out, as well as a review of the lakeside environment. Police are currently investigating whether the birds were killed intentionally by humans.

The league has increased the number of patrols in areas frequented by migratory birds and rolled out more-stringent rules for poultry markets.

About 300 km away from Beijing, the lake is a migratory stop for swans.