Baffled wildlife officials in China are investigating how hundreds of wild ducks were found dead in a frozen river when they should have been flying south for the winter.

The birds – a flock of ruddy shelducks (Tadorna ferruginea) – usually fly to warmer wetlands.

But horrified locals have reported how the entire population was wiped out in one night on the frozen Hun River, near Shenyang, Liaoning province.

Liaoning forestry department officials ruled out the cold weather as the birds face similar conditions every year.

And blood tests have confirmed that it was also not bird flu or another disease.

The ducks’ bodies have now been gathered up and are being tested to see if they were poisoned.

Officials said that the wild birds were not used to being fed but tests would confirm whether some form of poison had been used.

Online commentators said that they believed corn mixed with a toxic pesticide had been used to kill the ducks.

Forestry officials confirmed that corn was found on the ground at the scene but it is unclear if this was related to the widespread dead ducks.

They added that the ducks that died in a protected species and legal action would be taken if the killing turned out to be deliberate.