Chemicals used to make environmentally-friendly fuels could spell disaster for many British birds, experts said on Thursday.Scientists leading an 18-month probe into the sharp decline in sparrow and starling populations in Britain believe two of the chemical components of unleaded petrol - benzene and MTBE or methyl tertiary butyl ether - could be directly responsible. Billed as cleaner and greener, unleaded petrol uses the toxic chemicals to boost its octane-rating.