Wildlife experts are baffled by the sudden death of four adult swans and two cygnets on a short stretch of the Kennet and Avon Canal near Devizes.

They fear the protected birds may have been poisoned and others will die if action is not taken by DEFRA.

A pair of birds that nest every year near Devizes Marina next to the Hourglass pub have died and two of their four cygnets are also dead.

Two other adult swans and a number of ducks have been found dying on or near the canal.

Marilyn Boyes Korkis, the wildlife supervisor at Wiltshire Wildlife Hospital, is very worried about the deaths. She said: “Sometimes you get one dead swan but to get four adult birds and two cygnets dying in the space of a couple of weeks is very worrying.

“We have been called out a number of times to swans that are in a bad way but we have not been able to do anything for them. Our vet carried out an examination on one of the birds but apart from a swollen stomach could not find out what was wrong.

“This could have been caused by eating too much bread but that would not account for this number of deaths.

“There is nothing to suggest the birds have been attacked and they have not been shot, which has been suggested on social media.”

She said all of the birds had been found dead between Coate Bridge and The Bridge pub at Horton.

Other swans on the Caen Hill section of the canal had not been affected.

She has reported the deaths to DEFRA and hopes it will pay for full toxicology reports.

When the first incident was reported DEFRA said it could not become involved in a single death but Mrs Boyes Korkis hopes it will now take action.

Canal boat owner Paul Conner, who is moored by The Bridge pub at Horton, said: “Two weeks ago I saw a cygnet in distress and called the RSPCA and then the other day I saw a dead swan down towards the marina.

“It is upsetting that this is happening and I would like to know what is causing it.

“The swans are a big part of the wildlife on the canal. We get them tapping at our windows.”

Debbie Heritage, manager of Devizes Marina, said: “The deaths are upsetting and we would like to know what is causing them.”

A spokesman for DEFRA said the deaths could be down to avian botulism which had caused the death of a swan in Manchester.