A hiker had found the dead birds near a barbecue site and contacted the Agriculture, Fisheries and Conservation Department officers, who collected the birds for tests.

Agriculture department information officer Donald Lam Ping-kuen said the tests had been negative but more tests with other methods would also need to be conducted.

Dozens of discarded bird cages were found at a rubbish collection point in the country park, giving rise to suspicions the dead birds were freed by people believing the act of kindness would bring them luck.

'There have been cases in the past where birds which have been set free have died,' Mr Lam said. 'But it's only a suspicion, and since setting birds free is not a crime, there is not much we can do in terms of investigation.'

In February, dozens of sparrows were found dead in a car park near the Plover Cove Reservoir in the Pat Sin Leng park.

Sparrows are not usually kept as pets but traders capture them to sell to people wanting to release them.

Ornithologist Tsim Siu-tai, a conservation specialist for the Tai Po Environmental Association, said it was difficult for captive birds to survive the harsh wild environment. 'Birds are often packed like sardines into cages before being set free,' he said.

'Physically, the birds are weaker and mentally scared when set free. They also may not be fed enough. So if they are freed in bad weather, as was the case on Sunday, they may not have been able to survive.'

Conservationists have in the past called for an end to the release of captive creatures into the wild, where they usually cannot survive.