Twelve swans have died from a mystery infection at a lake in Merseyside.
The RSPCA has rescued another 30 critically ill swans from Marine Lake in Southport over the past couple of weeks.
They have been taken to the Stapeley Grange Wildlife Centre near Nantwich, in Cheshire.
RSPCA animal welfare officer Derek Hampson said the illnesses might have been caused by bacteria in the lake but further tests were being carried out.
"We are very concerned," Mr Hampson said.
Post-mortem tests are being carried out on the birds.
The ill birds are being treated with antibiotics and being closely monitored, Mr Hampson said.
The infection is not thought to be harmful to humans.