The mystery of Britain's disappearing house sparrows deepened yesterday with the publication of the most detailed sparrow distribution map to be compiled.
It shows that the birds are still plentiful in some areas of the UK and almost absent in others, with - as yet - no explanation. The map is the result of the biggest sparrow survey, based on garden observations made this year by 250,000 members of the Royal Society for the Protection of Birds (RSPB).
With such a huge number of responses, the survey's results are likely to be accurate. But far from answering the riddle, the map throws up fascinating new questions.
It shows, for example, that sparrow numbers are high in a broad belt along the east coast from the Wash to the Scottish border, extending into the east Midlands; yet on the opposite western side of the country, numbers are much lower. Is this because the east coast is drier than the west coast? Or that the east coast could receive birds across the North Sea from Europe, while the west could not? No one knows.
Similarly, while sparrows are shown to be plentiful in Lincolnshire, they are present in much lower numbers in north Norfolk. Is it a difference in agricultural practices or crop regimes? Again, no one knows.
The birds are scarce in the Pennines, the Lake District, Snowdonia and the Grampian mountains in Scotland. This is partly to be expected, because house sparrows cluster around human habitation. But they are much more common in the Cambrian mountains of mid-Wales, and on Exmoor.
Perhaps most intriguingly, sparrows are scarce in a broad swath of land that corresponds to the chalk belt of southern England, ranging from the Chilterns in Bedfordshire and Buckinghamshire, through the Berkshire and Hampshire downs, to Dorset. Is there something in chalk landscapes that Passer domesticus does not like? That remains to be discovered.
"It's a mystery. These questions are indeed intriguing, but as yet we simply do not know the answers," Richard Bashford, of the RSPB and organiser of the survey, said. "We will be doing further research to see if we can provide them."
RSPB scientists will be matching the map and survey with other data, such as types of land use, to see if any correspondences can be found. "Now that we've established the pattern of house sparrow distribution, we want to look in more detail at what factors are responsible for the variations," Mr Bashford said.
The survey confirms one fact - that big industrial cities and house sparrows don't seem to go together any more, although they were once a natural partnership.
London, Birmingham, Manchester, Liverpool and Glasgowhave lower numbers of sparrows than other, leafier urban centres such as Exeter, Cardiff and Aberdeen. In a sample of 27 cities quoted by the RSPB yesterday, London came bottom of the list, with an average of 4.53 sparrows per garden where they were present, compared with 9.41 sparrows per garden in Lincoln, the urban centre that scored highest. More than 15,000 people reported to the RSPB that they had no sparrows at all.
The survey also confirmed that more sparrows were likely to be seen in gardens where food was regularly put out for birds. An average of 7.3 sparrows were seen at homes where food was provided year-round, compared with an average of only 3.8 sparrows at homes where birds were not fed.
Numbers of Britain's house sparrows have more than halved in the past 25 years. Three years ago, The Independent offered a £5,000 prize for the first scientific paper to explain the reasons for the sparrow's disappearance from urban areas - a prize that remains unclaimed.
Denis Summers-Smith, the world authority on sparrows, thinks it may be caused by two main factors: the decline in insect numbers, which deprives young chicks of food, and a biological phenomenon known as the Allee effect - the inability of highly social animals to breed once their population drops below a certain level.
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