Government veterinary officers are working to establish how five swans died in the County Down area.
It is understood two more birds are ill. The dead birds' remains were found on private land in the Killyleagh area.
A spokesman for the Department of Agriculture and Rural Development (Dard) said the swans had been removed so that post-mortem exams could be carried out.
"This is the second case that has been brought to our attention," he said.
"The first case was investigated by Dard and the post mortem revealed lesions suggestive of bacterial infection."
"We would ask the public to refer to the Dard website or helpline for information on the types of bird deaths that we would like to be brought to our attention, so that Dard can monitor the situation and respond quickly."