An agricultural insecticide suspected of being behind the collapse of bee colonies in all parts of the world is apparently also triggering a fall in bird populations, a study indicates.

The pesticides, of a class dubbed neonicotinoids, attack a number of crop pests but they also leach into water and soil where they can kill insects other than their intended targets, Dutch researchers say.

That doesn't bode well for birds that depend on an insect food supply, they explained, adding that nine of the 15 bird species studied in the Netherlands only eat insects and all feed insects to their young.

In areas where the pesticides are widely used, "insects in general are dropping dead, and therefore there is less food for the birds available to feed their offspring," says ecologist Caspar Hallmann, lead author of a study published in Nature.

In some types of soils, neonicotinoids can persist for two to three years and accumulate to levels that represent a real risk to bird populations, the researchers said.

"If the concentrations are higher than 20 nanograms per liter in the environment, we found a reduction of 3.5 percent in local populations" researcher Hans de Kroon from the Netherlands' Radboud University says.

"In ten years it's a 35 percent reduction in the local population, it's really huge," he says. "It means the alarm bells are on straight away."

The study has brought a response from Bayer, the manufacturer of imidacloprid, one of the neonicotinoids analyzed in the study.

Bayer says it rejects the findings, saying the study hasn't proved a "causal link" joining the use of the pesticide and a decline in bird populations.

"Neonicotinoids have gone through an extensive risk assessment which has shown that they are safe to the environment when used responsibly according to the label instructions," a Bayer spokesman said.

Other researchers have come to the defense of the Dutch researchers, calling their study the first strong evidence that neonicotinoids are a risk to a greater number of species than just bees.

"If insectivorous birds are declining because of these chemicals, then other things that eat insects are going to be too," says Dave Goulson of the University of Sussex in Britain.

"We're also looking at bats and hedgehogs and shrews and fish and so on -- there's not much left, really," he says. "How much evidence do you need before you take action?"

Amid the controversy, the European Union has mandated a 2-year moratorium on the use of neonicotinoids on flowering crop varieties.