One woman's quest to solve mystery of the disappearing sparrows

Why are house sparrows disappearing from our towns and cities? It is Britain's greatest current wildlife mystery: Now, after a welter of theories and hypotheses ranging from attacks by magpies to the effects of climate change (and the offer of a £5,000 prize by The Independent), comes what may be the first step on the road to solving it.

A PhD student is finding that, in some circumstances, sparrows in built-up areas are failing to raise the second and third broods of chicks they need each summer to maintain their population levels, after deaths during the winter are taken into account. Unseen by the outside world, these midsummer chicks are dying in the nest.

They have not been attacked by predators; they have merely expired where they lay. They might have been struck by disease. They might have starved because the parent birds could not find enough insects. It is not yet known. But in certain locations they are dying in numbers large enough to cause the population to go into decline.

The new finding has been disclosed by a project of intense research into sparrow breeding organised in Leicester by Kate Vincent, 25, at the city's De Montfort University. Ms Vincent has spent the past three years putting up 619 sparrow nest-boxes across the city, which she visits every week in a van with a ladder on its roof. Leicester is as good a place as any to look at the reasons for sparrow decline; like London, Birmingham, Glasgow and many other large industrial cities, it has lost most of its birds from the city centre. Ms Vincent has had difficulty getting data on sparrows breeding in Leicester's central area, but from the suburbs and surrounding country, where her nest boxes have been widely used, her data has been voluminous and remarkable.

Her key finding last year was that, while in the countryside all the second and third nesting attempts she observed were successful, in the Leicester suburbs nearly half of them failed, with the chicks dying in the nest. Of 40 late broods in rural areas, all succeeded; of 30 in the suburbs, 14 (46 per cent) did not.

This year, although all figures are not in, something similar seems to be happening: 20 per cent of late broods so far observed in the countryside have failed; but of those in the suburbs, the failure rate is nearly double, at 38 per cent. The significance is the difference in the success rate between rural and built-up areas. House sparrows have declined in the countryside mainly due to more intense farming, and the decline seems to have stabilised. But the mystery is the much more recent steep decline in towns and cities, which is continuing.

Ms Vincent's findings, while not complete and not conclusive, suggest urban and suburban sparrows may be dropping in numbers because, although the chicks of the first, spring brood may be surviving to adulthood, the chicks of the second and third, midsummer broods are dying before leaving the nest.

This would fit in with the hypothesis advanced by the world expert on the house sparrow, Dr Denis Summers-Smith, who believes chicks may be dying because their parents cannot find enough insect food. Post-mortem tests Ms Vincent has had performed on chicks have been inconclusive; it is possible they were killed by infection, but it is also possible they starved. (One difficulty is that with one visit per nest box per week, dead chicks have often begun to decompose when they are found.)

To investigate the starvation theory and other possibilities, Ms Vincent has constructed a complex grid of observations around every box. They are directed to what insects the sparrows are feeding on and how abundant they are, and just what habitat is available to the birds within foraging range of 100 metres or so, concrete, weedy patches, mown lawn, hedges, allotments, and so forth.

Although little research has been done on exactly what insects sparrows feed to their chicks, she has already found, from analysis of the chicks' droppings, that different spec-ies seem to be fed to the early and late broods. The spring chicks appear to be fed on larger insects such as beetles and crane flies (daddy longlegs); the failing, midsummer chicks seem to be fed on smaller insects such as aphids.

Ms Vincent has thousands of pieces of data which she will spend this autumn and winter analysing. She is wary of drawing conclusions, but her Leicester observations seem to represent the first concrete step in addressing the mystery, which has fascinated and disturbed many, of how one of Britain's most familiar urban birds could suddenly decline.