Could the mystery of the vanishing sparrow be explained by the gardener's war on bugs?

The mysterious disappearance of the house sparrow from our towns and cities may all be down to greenfly - or a lack of them. The decline may well be due to a failure of the insect food supply for the young sparrow chicks, research by German scientists suggests.

The mysterious disappearance of the house sparrow from our towns and cities may all be down to greenfly - or a lack of them. The decline may well be due to a failure of the insect food supply for the young sparrow chicks, research by German scientists suggests.

Their conclusion comes in the first detailed study of the bird's decline in a major city - Hamburg, where the sparrow population is thought to have halved in the past 30 years.

The German report comes four months after The Independent launched its Save the Sparrow campaign, which aims to find the prime causes leading to the decline of the sparrow. Dozens of theories have been put forward by experts and readers, but the German study is the first conclusive evidence that a lack of food for chicks is a primary factor in the decline in urban areas.

The report shows that sparrows in the city centre are heavily dependent on one particular type of insect - aphids, the greenfly hated by gardeners - for feeding their young.

Nestlings that do not get enough aphids simply starve, and this may be happening regularly with the first of the three annual broods sparrows need to produce to maintain population levels.

In more than 300 letters to The Independent, readers have given possible theories, the most popular being the predation by magpies and sparrowhawks, formerly rural birds that have moved into urban areas, while other explanations proffered have included cats, grey squirrels, climate change, lack of nesting places and even disease caught from peanuts.

However, some readers with a knowledge of sparrow biology have suggested as the cause a failure of the food supply to the chicks. Although sparrows are seed-eating birds, for the first few days of their lives, perhaps the most vulnerable moment in their life cycle, the nestlings eat only insects.

The value of the German study is that it shows just how crucial the dependency is, and, secondly, the narrowness of the range of insects available to the birds, in Hamburg at least. Breeding can fail if the supply of one species falters.

The report, "House sparrows in Hamburg: population, habitat choice and threats", by Alexander Mitschke, Hilmar Rathjen and Sven Baumung, was produced last November for the Hamburg State Ornithological Protection Station.

It has been drawn to The Independent's attention by the marketing arm of Schwegler, a German manufacturer of nest boxes and other nature conservation products.

The study - prompted by the numbers of the German city house sparrow dropping steeply while other common birds such as blackbirds and blue tits remained constant - looked at sparrows in various control plots across the state of Hamburg and in particular in the Grossneumarkt area of the old city. Its conclusion is that the main population-limiting factor is the ability of the parent birds to find enough insects to feed their young, and that this often does not happen with the first brood in April.

For this brood, the parent birds are almost entirely dependent on aphids, which inhabit the lime and maple trees, and to a lesser extent the plane trees, in Hamburg's streets. For the second brood, the birds use ants, which feed on secretions the aphids produce, and so are also dependent on them. For the third brood, at the end of the summer, the sparrows are able to use flies.

About 40 per cent of Hamburg's sparrow population dies each year, the study found, so it is possible that first brood failure is the reason for the steady decline in numbers. The researchers do not know why the aphid population sometimes fails for the first brood, and they recommend more work should be done to find out, but they feel sure it is a major factor in the decline.

"The important thing is that they have problems in finding enough food to raise their young," Alexander Mitschke said. "They're very dependent on aphids. They need a lot of them because they're very small and they need them near the nest. House sparrows don't move long distances, most of the time searching for insects 50m from the nesting place.

"We don't know if the supply of aphids has halved in 30 years - we think there were probably more sorts of insects available then, but the greenery in which insects live has been greatly reduced as Hamburg has been built up."

The German study raises questions about the severe decline of the sparrow in Britain, in large urban areas estimated at more than 90 per cent.

First, although it is known that all young sparrow chicks are dependent on insect food, no recent detailed work has been done on exactly which insects the birds depend on in British cities - and whether the range is broad, or, as in Hamburg, now very narrow. In the past, sparrows have been known to take a wide range of insects, ranging from weevils to caterpillars.

Second, no work has been done on whether the insect population in British cities might have suffered a sudden and catastrophic crash, which would account for the sparrow's very sharp decline in British urban areas in the 1990s. And thirdly, no work has been done on what might have caused such an insect catastrophe, although pollution would be a major suspect.

There is no doubt, though, that researchers will soon start to look at these topics, and perhaps find a warning for ourselves that may be contained in the mysterious disappearance of our most common bird. Watch this space.