Lead-free petrol may be villain in mystery of demise of the world's most familiar bird

It would be a bitter irony indeed if the introduction of lead-free petrol were behind the disappearance from British cities of the house sparrow, the world's most familiar bird. But the circumstantial evidence is so strong, according to the world authority on sparrows, Denis Summers-Smith, that the hypothesis needs to be investigated urgently.

It would be a bitter irony indeed if the introduction of lead-free petrol were behind the disappearance from British cities of the house sparrow, the world's most familiar bird. But the circumstantial evidence is so strong, according to the world authority on sparrows, Denis Summers-Smith, that the hypothesis needs to be investigated urgently.

The removal of lead compounds from motor fuel was one of the biggest victories for the environment movement of the post-war period. The compounds, used in petrol to boost its burning efficiency, had been found to accumulate in children's bodies and gradually damage their brains. After a long struggle by green campaigners, an official decision was finally taken to phase out lead right across Europe. In 1988 unleaded petrol became available at British service stations; on January 1 this year it became compulsory.

But what replaced the lead? Substitute chemicals involved in boosting the petrol octane rating may have caused problems all of their own, Dr Summers-Smith believes, and may present the answer to the biggest environmental mystery of recent years - why the house sparrow has vanished from many of our big cities, virtually without trace.

Dr Summers-Smith is an engineering consultant and former senior scientific adviser to ICI. Now 79, he has been studying sparrows intensively for more than 50 years and has produced four books on the birds, including the standard monographs, The House Sparrow (1963) and The Sparrows (1988), as well as numerous papers in learned journals. His own sparrow database contains more than 5,000 items.

Two substances associated with unleaded petrol in particular, he feels, deserve urgent investigation - MTBE (methyl tertiary butyl ether), an additive, and benzene, a by-product of the refining. Both are toxic, of health concern, and known to cause cancers in animals.

There is a dearth of information on MTBE. The UK Petrol Industry Association cannot give a figure on how much unleaded petrol contains it, though it is probably less than 50 per cent.

The Department of Health, in an unpublished paper, signals health concerns about MTBE and says measurements should be made of concentrations in the air in Britain - none are done at present. California and Denmark currently plan to phase it out.

Benzene is better known and a proven carcinogen. It is thought to have become more prevalent with the uptake of the super unleaded petrol grade, which was widely bought by motorists in the early 1990s. The amount that can be present in petrol was cut from 5 per cent to 1 per cent by an EU directive that came into force on 1 January this year.

Dr Summers-Smith agrees that there is no scientific evidence as yet linking MTBE or benzene directly with house sparrows.

But he points out that the circumstantial evidence of a connection between sparrow decline and the introduction of unleaded petrol is strong.

For a start, he now believes that road traffic pollution is at the bottom of the sparrow mystery. He does so because of a remarkable discovery he has made after spending this summer analysing all the available sparrow population data: British sparrow populations have indeed collapsed in big cities - but not in small towns.

While he calculates the drop in cities such as London or Glasgow is of the order of 95 per cent, in small towns such as Crewkerne in Somerset or Guisborough in Cleveland the numbers have stayed virtually the same.

This is backed up by many of the letters in response to The Independent's Save the Sparrow campaign.

What cause of sparrow decline could operate selectively in cities but not small towns? Hardly any, Dr Summers-Smith says: not predation by cats and sparrowhawks, not disease, not lack of nesting places, not competition for food. All would have similar effects in small as well as in big conurbations.

Only road traffic pollution, he says, would be of a different order in cities, where a very much larger number of vehicles are present, often with engines idling, pumping out fumes. It would not have to affect the birds directly: it could, for example, cut down the number of insects they need to feed their very young chicks, which as German researchers have found, and the Independent reported last week, is the most vulnerable point in the sparrows' lifecycle.

Dr Summers-Smith then points out that the disappearance of the sparrow and the introduction of unleaded correlate closely in terms of time.

Although sparrows have been gradually declining for much of the past century, the real collapse of house sparrow populations in places such as London, much evidence suggests, is a phenomenon of the 1990s. In Kensington Gardens, for example, 885 birds were counted by ornithologists in 1948; in 1966 there were 642 birds, and in November 1975 there were 544. But in February 1995 there were only 46, and in July 2000 there were 12.

Lead-free petrol use is also a 1990s phenomenon. When it became available in Britain in 1988 at first it sold only in tiny amounts, but it represented nearly 90 per cent of total fuel sales by the end of last year, when leaded petrol was phased out completely.

"While the removal of lead from petrol was unquestionably right, could it be that it was at the cost of introducing other undesirable materials to the environment?" Dr Summers-Smith asks.

"There are at least two substances used in unleaded petrol that are potentially hazardous - MTBE and benzene. As the disappearance of the house sparrow from our large cities correlates with the introduction of unleaded petrol, the possibility that such substances are involved surely requires immediate investigation - if for no other reason than as an application of the precautionary principle."