Two birds have died following pollution of a pond in Horsham Park after a diesel spillage last week.
The birds - a duck and a moorhen - had earlier been rushed to a bird sanctuary for treatment.
But wildlife expert John Potts, from the organisation Swans and Friends Bird Rescue, said today that both had died.
And, he said, there were fears for a second moorhen which is currently in a poorly condition.
“I don’t hold out a lot of hope for it,” he said, “but time will tell.”
Meanwhile, a male and female swan are still being treated at the Swan Sanctuary in Shepperton after their feathers were covered in oil.
The pollution - the second within a year - happened on Wednesday last week when diesel spilled from the grounds of Horsham Hospital in Hurst Road into the park.
Contractors are still working on removal of the oil while investigations into what caused the spillage continue.
Horsham District Council, the Environment Agency and Southern Water have all been involved in investigating the cause and determining what action can be taken to make sure it does not happen again.
The pond area of the park remains closed to the public.