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A substance which has been covering birds on the south coast is an oil additive, a university has said.

The substance - found on seabirds off the south coast of Cornwall, Devon and Dorset, and up to West Sussex - has caused burning to birds.

Plymouth University said it was a form of polyisobutene (PIB), which was used as a lubricating additive in oils to improve performance.

It added it was informing the Maritime and Coastguard Agency and the RSPCA.

The substance was identified after analysis of a guillemot.

Chemical analyst Professor Steve Rowland, of the university's Centre for Chemical Sciences, said all the data it had gathered supported the conclusion.

He said: "It's very sticky and semi-solid, hence its acting on birds' feathers like a glue.

He added that it was also "difficult to biodegrade".

"It's quite big as a molecule, and quite difficult for bacteria and other weathering processes.

"Residues will probably stay around in the environment for some time, though hopefully not in this large mass that pollutes the birds."

The pollutant has affected hundreds of seabirds washed up since last week.

Birds have been found with the substance congealed on their feathers, reducing their waterproof coating and ability to stay warm and afloat at sea.

Prof Rowland said he was aware of only one other incident of the "relatively common" substance being spilled in 1994.

But despite spills being "comparatively rare", he added that it would be difficult to trace the source.

He said: "It's probably transported around the world.

"Whether we can find out if it was spilled would require a cargo sample to compare it with."

More than 300 birds, mostly guillemots but also some razorbills, have been treated at the RSPCA West Hatch centre near Taunton, Somerset.