Staff at the Royal Society for the Prevention of Cruelty to Animals center in West Hatch say they have taken in about 170 live birds covered in the oily substance since Tuesday, The Daily Telegraph reported.
"It is like a silicone sealer. The best way I can think to describe it is 'sticky Vaseline,'" said RSPCA deputy chief inspector John Pollock.
Volunteers at the center are working to clean the surviving birds while RSPCA animal collection officers said they have seen hundreds of dead birds washed up along the southern coastline.
The Environment Agency is looking into what the substance covering the birds is and where it may have come from, the newspaper said.
The agency took samples from the affected water to establish the cause of the pollution.
"The results show that it is a refined, mineral-based oil mixture, but not from an animal or vegetable-based oil, which rules out palm oil," an Environment Agency spokesman said.