The decline of the house sparrow, one of Britain's best known birds, could be down to a lack of insects and spiders during the summer months, experts say.
Conservationists are worried about the bird's decline (Image: Laurie Campbell and rspb-images.com)
A De Montfort University team claims adult sparrows are struggling to find enough food to feed their chicks in the early stages of their lives.
UK bird charity RSPB said measures that boosted insect numbers could also help arrest the sparrows' decline.
This included growing deciduous trees and shrubs and cutting insecticide use.
In August 2002, the house sparrow was added to the Red Data list of bird species of conservation concern because their decline had been more than 50% in the last 25 years.
Between 1977 and 2000, house sparrow (Passer domesticus) numbers in the UK declined by 65%.
Kate Vincent, of De Montfort University in Leicester, put up more than 600 nestboxes around the East Midlands city to study the birds' breeding success, chick condition, diet and feeding habits.
Ms Vincent found that chicks were more likely to starve if their diet contained a high proportion of vegetable matter or ants, and less likely to starve if their diet contained a high proportion of spiders.
More chicks were dying in the sparrows' second brood of the year, an observation that may help to explain the overall population decline.
Because of the high chick mortality rate during winter, the species needs two or three broods every year to keep the population at least level. The De Montfort researcher found there was an 80% success rate in the first brood, but a 65% success rate in the second.
"While we can't pinpoint one simple cause of sparrows' decline, food limitation during the breeding season does have a negative impact on nestling survival rates," Ms Vincent explained.
She added that the combined effects of relatively high rates of chick starvation and low body masses at fledging (which influences survival after fledging) observed in suburban localities are large enough to result in rapid population declines.
Thus far, there is no evidence that the birds' invertebrate prey have fallen in abundance in urban and suburban of Britain.
But Kate Vincent says it provides a plausible mechanism for the observed declines in populations of the birds.
Abundance of insect and spiders seems to affect survival (Image: Ray Kennedy and rspb-images.com)
Dr Will Peach, senior biologist at the Royal Society for the Protection of Birds (RSPB), said: "Dr Vincent's study has clearly demonstrated that a lack of insects in suburbia during summer prevents house sparrows rearing their young.
"Although we are not sure about the exact causes of the population decline, any measures that boost insect numbers in gardens should help nesting sparrows."