More birds have been washed up on the south coast of Cornwall and Devon covered in a sticky substance.
Twelve guillemots have been collected on beaches from Mevagissey to Whitsand Bay and Plymouth.
The RSPCA said they were covered in "what appears to be the same white, sticky substance" that killed hundreds of sea birds in February.
A spokesman said: "There is no indication of where the pollution is coming from."
He added: "Five of the birds have been taken to the RSPCA West Hatch wildlife centre in Taunton, Somerset, where unfortunately due to their extremely poor condition, they have been put to sleep."
In February more than 300 birds, mainly guillemots and some razorbills, were taken in by the RSPCA after being found covered in glue-like polyisobutene (PIB).
The birds were found in Dorset, Cornwall, Hampshire and Sussex.
Staff used washing up liquid and margarine to clean the animals.
The RSPB has called for PIB, which can be released legally under certain conditions, to be reclassified and discharges outlawed.
The Maritime and Coastguard Agency launched an investigation into the first spill, but has since said it had been unable to trace the source of the pollution and confirmed it had closed the investigation.