Mystery substance could kill 'many more' birds
1 February 2013 Last updated at 22:34 GMT
Wildlife experts have warned many more birds could be affected by the waxy substance found on guillemots washed up in the south west.
Hundreds of seabirds were found on beaches from Sussex to Cornwall on Thursday, many at Portland in Dorset.
Scientists are trying to establish the nature and source of the substance, which is thought to be a refined mineral oil.
The BBC's Robert Hall reports.