It's taken eight-and-a-half years – but The Independent's £5,000 prize for explaining the disappearance of the house sparrow from our towns and cities finally has a serious entry, with a serious theory.

Insect decline, featured prominently in this newspaper last Saturday, is offered as the reason for the biggest bird mystery of modern times by a group of four scientists from the Royal Society for the Protection of Birds (RSPB), De Montfort University in Leicester and Natural England, the Government's wildlife agency.

Their theory, put forward in a scientific paper to be published in a forthcoming issue of the journal Animal Conservation, is based on intensive research in Leicester, showing that sparrow chicks were starving in their nests because their parents could not find enough insects to feed them. So many chicks were dying that the birds' population level as a whole was declining.

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The paper has been entered for The Independent's prize, which was reported around the world when it was announced on 16 May 2000, as the start of a campaign to Save The Sparrow.

One of the authors, Dr Kate Vincent, who carried out the research on which the theory is based, said: "If we were successful, given the statuses of the collaborating organisations, we feel that any prize money received should be spent on further research or conservation work on house sparrows. From The Independent's point of view it would be a natural progression to know that your Save The Sparrow campaign prize money would be engineering further sparrow research, which we feel would be something to celebrate."

The £5,000 prize was offered for a peer-reviewed paper published in a scientific journal, which – in the opinion of our referees – would account for the disappearance of the house sparrow, Passer domesticus, from towns and cities in Britain. The referees are the RSPB, the British Trust for Ornithology (BTO), and Dr Denis Summers-Smith, an internationally renowned expert on sparrows. (It was stated at the time that researchers from the RSPB and BTO would not be precluded from entering.) The referees will now be considering the paper to see if it does indeed account for a remarkable wildlife enigma.

House sparrows in Britain have declined by 68 per cent since 1977, but the decline has been overwhelmingly an urban one. Although still relatively plentiful in small towns in the countryside and by the sea, in many major conurbations, sparrows have disappeared. Numbers started falling in cities in the mid-1980s and the species has virtually vanished from central London – for example, St James's Park holds all the common garden birds such as blue tits, robins and blackbirds, but sparrows, which were once plentiful, died out in the park in the late 1990s. There was no obvious cause. House sparrows are also disappearing from Bristol, Edinburgh and Dublin, as well as Hamburg, Prague and Moscow but curiously, they are faring better in Paris and Berlin.

When The Independent launched its campaign, many potential reasons for the decline were suggested by readers, which included increased predation by cats, magpies and sparrowhawks (all of which have increased in our cities); disease contracted from bird food such as peanuts; increased use of pesticides; collective suicide; radiation from the Chernobyl nuclear accident in the former USSR; the disappearance of sparrow nesting places as houses were modernised and gardens were tidied up or concreted over as car ports, and a decline in insects. (Although adult sparrows are seed-eating birds, the young need insect food in the first few days of their lives).

The last two potential causes – lack of nesting places and insect shortage – have always seemed the most likely (although many people blame magpies and other predators for declines in small birds, both the RSPB and the BTO say this is not borne out by the figures).

A leading proponent of the insect shortage theory was Dr Summers-Smith, a retired engineer from Guisborough in Cleveland, and the author of the standard monograph on the house sparrow, and several other sparrow books. He felt that chick starvation might well be the cause of the fall in numbers as a whole, although he was unable to prove it.

This appears to have been borne out in the new paper. The lead author, Dr Will Peach from the RSPB, said: "Each pair of house sparrows must rear at least five chicks every year to stop their numbers falling.

"But in our study, too many chicks were starving in their nests. Others were fledging [leaving the nest] but were too weak to live for much longer than that. If the birds nested in areas rich in insects, they did much better.

"Where there were few insects, young house sparrows were likely to die. Young house sparrows need insects rather than seeds, peanuts or bread to survive."

Dr Vincent, then of De Montfort University in Leicester, said: "This is one of the most mysterious and complex declines of a species in recent years. The study highlights that sparrow chicks are hatching but they aren't surviving.

"This is partly down to the loss of green spaces within British cities through development on green space, tree removal and the conversion of front gardens for parking. The loss of deciduous greenery within urban areas may have made life much more difficult for birds like house sparrows that need large numbers of insects to feed their young."

Phil Grice, senior ornithologist at Natural England, said: "This study highlights the importance of using native varieties of plants in our urban green spaces which, in turn, support large numbers of insects that are important in the diet of house sparrows and a range of other birds that we love to see in our gardens".

The paper, Reproductive success of house sparrows along an urban gradient, by W J Peach, K E Vincent, J A Fowler and P V Grice, is now being sent to our referees and we shall report soon on their verdict.