TESTS on swans found suffering from a mystery illness at a Weymouth nature reserve have not been able to reveal what caused their deaths.

Half the resident herd at Radipole Lake has been struck by the disease and 14 have died.

The RSPB previously reported that 16 had died but it has since emerged that two have survived and are being cared for at the RSPCA centre in West Hatch.

While bird flu was immediately ruled out by tests, post-mortem results revealed today have proven inconclusive.

RSPB spokesman Tony Whitehead said: "It is possible that the birds simply succumbed to a naturally occurring pathogen.

"Avian botulism for instance is known to kill thousands of birds, particularly after spells of warm weather.

"However it is difficult to establish to botulism post-mortem as the toxins are often no longer in the system."

The illness first struck in September and as there have been no deaths in the past two weeks, Mr Whitehead said further tests to establish what caused the outbreak will not be carried out.

The neighbouring reserve at Abbotsbury has been unaffected and only swans have come down with the illness.

Luke Phillips, information officer at the Radipole Lake Nature Reserve, said it had been a shocking outbreak.

This video shows him feeding some of the healthy swans remaining at the lake.

He said: "The ill birds were really lethargic, quite weak and exhausted but there were no signs of physical injury.

"They do hide illnesses quite well, so if they show any signs you know they are quite poorly."

Mr Phillips added: "It really has had an effect on the herd. You normally see so many on the lake between the centre and Westham Bridge. Our resident herd is about 30 to 35 and now we only have half of that left.

"I'd like to thank the members of the public for their concern, and also the RSPCA, who have done a great job in trying to help the affected birds."

There is no risk to humans from the disease but anyone who finds dead or sick swans at Radipole Lake is asked to contact the site wardens on 01305 778313.