Scanning the sky with his binoculars, he searches carefully for any sign of movement: the steady beat of a blackbird's wings, the fluttering of a flock of starlings. It has been a week now since he saw the starlings: just four of them flitting from tree to tree, feasting on the autumn berries.

Birds are a real rarity these days. In his boyhood, he recalls, he would watch the acrobatics of entire flocks as they ducked and dived after insects. But now the skies are silent, barring the hum of the odd airplane. Turning back to his fruit and vegetable patch, he continues the laborious task of pollinating the raspberry plants by hand, gently brushing pollen onto the slender stigmas inside the flowers. In the past, bees, wasps, butterflies and flies would have done this job for him; nowadays such insects are likewise a rarity. Farmers instead resort to robot bees to pollinate their crops: tiny motors, encased in fuzzy fabric, which hover from flower to flower.

Will this bleak outlook be a reality for future generations? It is nearly 50 years since Rachel Carson wrote Silent Spring, the book that warned of environmental damage the pesticide DDT was causing. Today, DDT use is banned except in exceptional circumstances, yet we still don't seem to have taken on board Carson's fundamental message.

According to Henk Tennekes, a researcher at the Experimental Toxicology Services in Zutphen, the Netherlands, the threat of DDT has been superseded by a relatively new class of insecticide, known as the neonicotinoids. In his book The Systemic Insecticides: A Disaster in the Making, published this month, Tennekes draws all the evidence together, to make the case that neonicotinoids are causing a catastrophe in the insect world, which is having a knock-on effect for many of our birds.

Already, in many areas, the skies are much quieter than they used to be. All over Europe, many species of bird have suffered a population crash. Spotting a house sparrow, common swift or a flock of starlings used to be unremarkable, but today they are a more of an unusual sight. Since 1977, Britain's house-sparrow population has shrunk by 68 per cent.

The common swift has suffered a 41 per cent fall in numbers since 1994, and the starling 26 per cent. The story is similar for woodland birds (such as the spotted flycatcher, willow tit and wood warbler), and farmland birds (including the northern lapwing, snipe, curlew, redshank and song thrush).

Ornithologists have been trying desperately to work out what is behind these rapid declines. Urban development, hermetically sealed houses and barns, designer gardens and changing farming practices have all been blamed, but exactly why these birds have fallen from the skies is still largely unexplained.

However, Tennekes thinks there may be a simple reason. "The evidence shows that the bird species suffering massive decline since the 1990s rely on insects for their diet," he says. He believes that the insect world is no longer thriving, and that birds that feed on insects are short on food.

So what has happened to all the insects? In the Nineties, a new class of insecticide – the neonicotinoids – was introduced. Beekeepers were the first people to notice a problem, as their bees began to desert their hives and die, a phenomenon known as Colony Collapse Disorder (CCD).

The first cases were in France in 1994, but the epidemic quickly fanned out across Europe, and by 2006 CCD reached the US too. Between 2006 and 2009 one third of American beekeepers reported cases of colony collapse. Aside from the loss of revenue in honey sales, this is worrying news because honey bees are one of the world's most important pollinators, and 35 per cent of agricultural crops rely on pollinators.

As a service, pollination is worth an estimated £440m a year to the UK economy and a staggering $15bn (£9.3bn) to US farmers. And it isn't just the Western world that is affected: in China the lack of bees has become so serious that farmers in some regions are already resorting to pollinating their crops by hand.

Controversy has swirled around the issue, with everything from mobile phones to GM crops being held to blame. The key contenders include parasites, viral and fungal infections, and insecticides.

Last month the problem appeared to have been cracked, when a group of US scientists published a paper in the online journal PLoS One which indicated that CCD was caused by the interaction between a virus, the invertebrate iridescent virus, and a fungus known as Nosema apis ( http://ind.pn/9NKzPD).

But since then it has emerged that the study's lead author, Jerry Bromenshenk, has in recent years received a research grant from Bayer Crop Science (a leading manufacturer of neonicotinoid insecticides) to study bee pollination. Bromenshenk has, however, said that no Bayer funds were used in the earlier study. Jeroen van der Sluijs, of the Netherlands' Utrecht University, doesn't doubt Bromenshenk's findings, but says they don't address the key issue: "Previous research has shown that exposure to neonicotinoids makes colonies more prone to the Nosema fungus and virus infections."

If that is so, then neonicotinoid insecticides could be the root cause of the problem. But why are they so much worse than other insecticides?

"Neonicotinoids are revolutionary because they are put inside seeds and permeate the whole plant because they are water-soluble (which is why they are called systemic insecticides). Any insect that feeds on the crop dies," explains Tennekes.

Even small doses can kill. Recent research, carried out on honey bees in the lab, showed that these insecticides build up in the central nervous system of the insect, so that very small doses over a long time period can have a fatal effect. The reason that neonicotinoids can have such a powerful long-term effect is down to the way they work – binding irreversibly to receptors in the central nervous systems of insects.

"An insect has a limited amount of such receptors. The damage is cumulative: with every exposure, more receptors are blocked, until the damage is so big that the insect cannot function any more and dies," explains van der Sluijs.

And unfortunately the robust nature of neonicotinoids means that they can travel far beyond the crops they were used to treat. "Neonicotinoids are water-soluble and mobile in soil. They can be washed out of soils and into surface and groundwater – as we've seen in the Netherlands since 2004. As a result, neonicotinoids are probably readily taken up by wild plants as well, and in this way spread throughout nature, causing irreversible damage to non-target insects," says Tennekes.

Many scientists now agree that there is strong evidence to suggest that neonicotinoid insecticides are damaging to bees. But what about the other insects? Are they being poisoned in the same way? "It is very difficult to prove, but I believe that most insects will have declined since the introduction of neonicotinoids in the 1990s. The problem is that we are not really interested in insects, apart from bees (because we need them) and butterflies (because they are pretty). However, the few insect species that we monitor closely indicate massive decline," says Tennekes.

A new PhD thesis goes some way to backing up Tennekes's claim. This year, Tessa van Dijk at Utrecht University demonstrated a strong link between increased pollution levels and a reduced presence of insects, and especially flying insects, in regions of the Netherlands where residues of neonicotinoids are high.

Others agree that Tennekes may be onto something. "It is a plausible theory that birds that feed on insects, or that feed their chicks on insects only, will suffer from insect decline. But much more data are needed to understand how big the role of neonicotinoids is," says van der Sluijs.

Nigel Raine, a bee expert from Royal Holloway, University of London, concurs. "There is not yet enough evidence to show that neonicotinoid insecticides are environmentally safe in the longer term. But if it can be proved that they are causing a decline in insects, it is reasonable to assume a link to a decline in the bird species that eat insects."

Some argue, however, that the story is unlikely to be so simple. "Bird decline started before neonicotinoids hit the scene. Like so many things, the decline of bird populations is almost certainly multifactorial, involving pesticides, habitat loss and many other variables," says Gard Otis, an entomologist at the University of Guelph in Ontario, Canada.

Nonetheless, some countries have already begun to take action. In 2008 the German, Italian and Slovenian authorities imposed a ban on the use of two types of neonicotinoid insecticides on maize. Meanwhile France has had a ban in place since 1999, on a neonicotinoid insecticide used to dress sunflower seeds.

But for Tennekes the only solution is a global ban. "Neonicotinoids act like chemical carcinogens, for which there are no safe levels of exposure. The message is that we must act quickly and ban these compounds, to avoid a catastrophe," he says.