The diesel leaked through a culvert into Bumble Hole Pool, at Bumble Hole Nature Reserve, in Netherton, Dudley.
The RSPCA has been on the scene cleaning birds, while the Environment Agency is working to trace the source of the spillage
Photographs showed a red substance lying on the surface of the water while birds were coated with a dark substance.
Make-shift barriers have been placed within the lake to contain the spillage and prevent any more pollution. Absorbent pads have also been used.
The RSPCA said eight Canada geese, one swan and ducks have been affected.
A spokesman for the charity said: "Some are really badly oiled and will need a couple of washes. Some of the patients are really sick but our team will do the best they can, as they always do."
Councillor Karen Shakespeare, cabinet member for public realm at Dudley Council, said: "We became aware earlier today that diesel was getting into Bumble Hole pool via a culvert.
"We informed the Environment Agency which will investigate to find the source and work to clean and prevent further pollution of the pool.
"We also contacted the RSPCA who have attended to look after and clean the birds."
The spillage was first reported to authorities yesterday.
A spokesman for the Environment Agency said: "We can confirm there has been a discharge of a diesel substance at the Bumble Hole Nature Reserve in Netherton.
"Our officers are investigating the source of the contamination and working with Severn Trent Water and Dudley Council to contain the pollution and prevent it from spreading.
"If anyone sees any further pollution issues they should contact our 24/7 hotline on 0800807060."
The Environment Agency is being supported by Severn Trent Water, which looks after water networks in the area.
A spokesman for Severn Trent Water added: "We’ve got teams on site who’ve put booms and pads out to stop any further pollutants getting into or out of the lake.
"We’re also working with the Environment Agency to try and trace the source of the pollution."
The RSPCA have been approached for comment.