About 20 birds have been caught up in an oil spill in a Hertfordshire river which the RSPCA described as "severe".
A swan, glebe and 15 mallard ducks were rescued from Walsworth Common, Hitchin, on Tuesday.
RSPCA inspectors returned to the site to catch at least five more birds not so badly affected.
The birds were washed and fed carbon to get rid of any oil they had swallowed and have been recovering at a wildlife hospital, an RSPCA spokeswoman said.
Peter Green, one of the RSPCA inspectors at the scene, said: "It was quite a serious mess really. It's pretty unusual for it to be so bad.
"It is not at all good for the birds to be covered in oil. It stops them flying and means they are not waterproof so struggle to float on water.
"The birds were constantly trying to clean off the oil also. They were exhausting themselves with the effort, plus there are all kinds of issues with them ingesting oil.
"I was quite worried about them."
The Environment Agency said it believed the spill was caused by someone pouring used cooking oil down a manhole.
A contractor was called in to clean the river up on Wednesday, an agency spokeswoman said.