A new generation of environmentally-friendly chemicals is increasingly suspected of being one of the main causes of the sharp dive in the numbers of the birds, whose arrival has been greeted since the time of ancient Greece as the harbinger of good weather.
Traditionally they have been seen as specially sacred birds: it was thought to be unlucky to kill them and a sign of great ill omen if they disappeared.
Swallows particularly congregate near cattle, feeding off the insects that fly around them. But cattle farming has declined in many parts of the country, and the effect of this on the birds, it is suspected, may be compounded by new "green" chemicals used to protect the animals.
The chemicals appear to be an environmentalist's dream, doing no harm to livestock or man, yet being extremely efficient at killing pests. But - though nothing is proven, and pesticide manufacturers contest the suggestion - ornithologists fear that, in doing so, they may be depriving swallows of their food.
Two main chemicals are under suspicion. The first are artificial pyrethroids, which mimic the properties of pyrethrum, a natural pesticide which has long been used by organic gardeners. Plastic tags, impregnated with a pyrethroid, are attached to the ears of cattle and slowly spread the chemical throughout the thin oily layer that covers the animal's skin, killing any fly that settles on it.
The second wonder chemical is ivermectin, which is extremely effective at killing worms and parasites and is given to young cattle. Work by researchers at the Joint Nature Conservation Committee, the Government's official wildlife watchdog for the whole of Britain, has suggested that the chemical passes through the animals and sterilises their cow pats - which are fertile breeding grounds for flies and other insects.The manufacturer of ivermectin, MSD AgVet, strenuously denies that it has any effect on wildlife.
MSD AgVet cites research partly carried out by another company in the same group, Merck AgVet in the United States, which accepts that the pesticide does affect the number of insects in cow pats but adds that this is only temporary. And it says that as only young cattle are treated with the pesticide there should always be unaffected pats from other animals nearby.
Ornithologists are calling for more research into the pesticides to see if they are indeed affecting swallows and other birds. Dr Chris Mead, of the BTO, said that house martins are declining as well as swallows, and that some villages in the West Country and Midlands have lost them altogether.
He accepts that the chemicals are exceptionally safe for both humans and livestock and said that they seem to be so much better for cattle than alternatives.
He added: "I am sure you could butter your toast with these chemicals and be unharmed, but that is not the effect we are looking for. Both sorts of chemicals could be affecting bird species. They could be a very important source of harm to wildlife."
The Royal Society for the Protection of Birds said last week that it was aware of the concerns about the chemicals, took them seriously and wanted to know more about what effects they make have.
Mr Vic Simpson, a veterinary surgeon from Cornwall who has taken a special interest in the pesticides, commented: "They are nice safe insecticides and 'green' compounds. They do a good job, but it may almost be too good a job. It just shows that anything may have side-effects."
The attraction of the theory about the effects of the new green chemicals is that ornithologists say that no other explanation has been found that fully explains the swallows' precipitous decline.
Some have suggested that drought in the Sahelian belt of Africa, south of the Sahara, was to blame. The BTO says it does affect sand martins which winter in the area, but not swallows, which spend winter in southern Africa.
A decline in ponds, which provide mud for swallows' nests, and modern "bird-proof" farm buildings which stop them nesting in them have also been blamed.
But the BTO says that the swallows still succeed in finding mud and nesting places elsewhere.Reuse content