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.
They don't know where the substance has come from.