The swans were affected by an oily substance

It is still not clear what caused the oily damage to their feathers.

Wildlife officers took 140 swans off the pool last month after their feathers became covered in an oily substance.

It meant they were no longer waterproof and they were in danger of catching hypothermia.

Two of the birds died but the rest have been cared for at the Wierfield Wildlife Hospital in Lincoln and the Norfolk RSPCA hospital.

The Environment Agency has carried out tests on both the water and the birds and say the illness remains a mystery.