The cause of the deaths of 48 swans in or near a wildfowl reserve in Co Wexford may be lead poisoning, according to the National Parks and Wildlife Service.
Some 15 whooper swans were found to have died by December 24th, leading to fears that the birds might have died from avian flu.
However, scientists from the Department of Agriculture were able to establish that the cause of the deaths of the birds, who arrive from Iceland each year, was not avian flu.
The death toll has continued to rise even though tests have revealed that the swans did not have an infectious disease.
Whooper swans breed in Iceland and north-west Europe and migrate to western Europe for winter.
They begin arriving in October, with peak numbers coming in mid-winter. They begin leaving for their breeding grounds in March and April.
Some 14,000 swans spend winter in Ireland and these are distributed in about 220 flocks throughout the island. Most of the whooper swans that winter in Ireland come from the Icelandic breeding population.
Postmortem results have suggested the most likely cause of death is lead poisoning. However the wildlife service admits the reason why so many birds are suddenly dying from lead poisoning is a mystery.
All swans ingest a certain quantity of lead as they swallow stones to help their digestion, and some whooper swans would be expected to die after the long migration from Iceland.
However the high number of deaths has caused concern in the wildlife service because the source of the lead poisoning has not been identified.
Wildlife service officers searched the North Slob area, including those areas of Wexford harbour abutting the North Slob. Yet the number of dead swans rose to 32 by December 30th, with most of them located on or in the vicinity of the Wexford Wildfowl Reserve.
The numbers have since continued to increase, with a small number of dead whooper swans found outside the North Slob at Tacumshin, on the south coast of Co Wexford, and on the South Slob, which is located on the south side of Wexford harbour.
Sickly birds have also been observed at Tacumshin but on-going tests for avian flu or other infectious diseases of concern to the poultry industry, continue to prove negative.
Minister for the Environment Dick Roche, who is responsible for the wildlife service, said the cause of the deaths was not an infectious disease, which would be a relief to poultry farmers.
He said the wildlife service and the Department of Agriculture would continue to investigate.
