By the time Pierre Mineau flashes the image of the dead Swainson's Hawks up on the screen, he has already made it very clear that pesticide use is hardly a new problem. After all, humans probably began looking for the best way to kill pests soon after they started growing crops 10,000 years ago.

A research scientists with the Canadian Wildlife Service, Mineau is mainly concerned with the many and varied ways that pesticides kill birds, but he builds to this point. During a recent lecture in Whitehorse, he explained how people experimented with natural means of pest control even in medieval times, when sulfur was used to fumigate crops.

The use of synthetic pesticides really began in the 1930s, after a Swiss chemist named Paul Müller discovered DDT. In World War 2 DDT was seen as a wonder chemical, and was sprayed and dusted on the soldiers and in their trenches to kill the body lice that spread typhus.

In 1948 Müller received the Nobel Prize for his efforts. Fast forward to 1967, when Rachel Carson's book "Silent Spring" was published. This ground-breaking book alerted the world to problems with pesticide use, warning that the continued use of pesticides would doom migratory birds.

DDT caused birds to lay eggs with thinner shells, so often the eggs would be crushed in the nest. Peregrines were one of the many species of birds around the world whose numbers plummeted because of this problem. "Silent Spring", which is credited with kicking off the modern environmental movement, also started the fight to restrict the use of organochlorines such as DDT.

But the dead hawks shown in Mineau's presentation were killed by another class of synthetic pesticides, organophosphates, and this slaughter marked another milestone in our understanding of the grievous ways that pesticides can cause harm.

In the early 1990s, North American biologists watched with dismay as fewer and fewer Swainson's Hawks returned every year to their breeding grounds on the prairies of western North America. At that time no one was even certain where the birds spent their winters.

Eventually, with the help of radio transmitters, the birds were tracked to the Pampas region of Argentina. Many farms dot the grasslands there, and the hawks gather in huge flocks in isolated stands of trees bordering the fields. During the day they gorge themselves on grasshoppers, one of their favourite foods.

Unfortunately for the birds, farmers were using an organophosphate called monocrotophos to kill the grasshoppers. Even though the pesticide was not registered for use either on grasshoppers or on alfalfa, the main crop in the area, there was no clearly-articulated prohibition against those uses.

The pesticide was so deadly for the hawks that they were literally falling from the trees as they roosted during the night. During the winter of 1995-1996, it was estimated that as many as 20,000 hawks died in the area.

In his role as head of the Pesticide Section of the National Wildlife Research Centre, Mineau helped advise the farmers and the Argentinean government on less lethal ways to control grasshoppers. The American government pumped money into the program, financing a surveillance and education program on the use of alternative pesticides.

This program worked, and fewer and fewer dead hawks were found in the Argentinian grasslands. But other incidents still occurred. In 1997 about 100,000 doves were killed in one area after a farmer soaked his seeds in monocrotophos.

Even though this pesticide is now banned in some countries, world-wide about 60 million hectares are still treated with it every year. Monocrotophos is still used widely in Central and South American countries, where many Canadian migratory birds spend their winters.

"More worrisome is the fact that monocrotophos is the second most popular insecticide in the world in terms of tonnage," says Mineau. "This does give us cause to worry about the fate of our migratory birds."

Pierre Mineau is the head of the Pesticide Section of the National Wildlife Research Centre with the Canadian Wildlife Service. He can be contacted at Pierre.Mineau@ec.gc.ca. For information on how pesticides affect birds in North America, see your Yukon column 196, "Farms can be toxic for birds."