Hundreds of seabirds have been washed up along the south coast between Dorset and Cornwall.
Coastguards have said birds have been found covered in a sticky, glue-like substance.
The RSPB and RSPCA have launched a rescue operation after around 100 birds were washed ashore.
Some birds have died but rescuers are trying to save others.
Samples have been sent for tests.
The birds are being treated with a mix of washing up liquid and margarine.
The oil in the margarine helps get the grease of the bird's feathers without stripping their waterproof coating.
People are being told not to pick up the birds and to avoid walking dogs on the beaches.