"A dupe of the radical Greens!" "A disingenuous corporate stooge!" What could provoke such contradictory ad hominem attacks on your humble science correspondent? My simple observation in last week's column on the 30th anniversary of the Endangered Species Act: "It is generally acknowledged that banning DDT, which thinned bird's eggshells, brought back the bald eagle, the peregrine falcon, and the brown pelican." The controversy over the pesticide DDT and bird eggshell thinning is still going strong more than 30 years after the pesticide was banned in the United States.

DDT and eggshell thinning and the link between them is an ongoing subject of political controversy, if not necessarily scientific controversy. Public concern over DDT can be traced back to Rachel Carson's 1962 book Silent Spring, beloved of environmentalists for blaming mankind's carelessness for unprecedented destruction of nature, and sneered at by many free-marketers for triggering lots of unwarranted fears and environmental law-making.

The situation regarding DDT and eggshells is not as straightforward as one might like. Science always deals with provisional conclusions. The first thing that one notices when plunging into the relevant scientific literature is how dated most of the eggshell-thinning research is. Most of the significant articles were published before 1980. "[The issue] kind of died out. There's a general lack of interest," agrees Daniel W. Anderson. Anderson, now at the Department of Wildlife, Fish, and Conservation Biology at the University of California-Davis, was one of the original researchers on eggshell thinning. He blames the lack of new research on a lack of funding. Besides, Anderson observes, "the questions about eggshell thinning were pretty well answered, so people moved onto other things."

Rachel Carson cited early anecdotal reports of various birds either dying of acute DDT poisoning (usually by eating poisoned insects) or experiencing reproductive problems, thus giving her her title conceit. No birds singing, a silent spring, get it? Her book was a popular phenomenon, and not surprisingly her claims drew the attention of a lot of researchers.

The DDT/eggshell thinning bandwagon got really rolling with two scientific articles. The first study, "Decrease in Eggshell Weight in Certain Birds of Prey," by British Nature Conservancy researcher D.A. Ratcliffe, was published in Nature on July 8, 1967. Ratcliffe claimed that the incidence of broken eggs in nests of peregrine falcons, sparrowhawks, and golden eagles had increased considerably since 1950. He compared eggshells collected before 1946 with eggshells collected afterward, and found that post-1946 peregrine falcon eggshells weighed 19 percent less; sparrowhawks' weighed 24 percent less; and golden eagles' 8 percent less. Ratcliffe dismissed lack of food and radioactive contamination as explanations for the thinning, but noted "some physiological change evidently followed a widespread and pervasive environmental change around 1945-1947... For the species examined, frequency of egg-breakage, scale of decrease in eggshell weight, subsequent status of breeding population, and exposure to persistent organic pesticides are correlated. The possibility that these phenomena are links in a causal chain is being investigated," he concluded.

Those British results were soon bolstered by the study "Chlorinated Hydrocarbons and Eggshell Changes in Raptorial and Fish-Eating Birds," published in an October 1968 issue of Science, and authored by Daniel Anderson and Joseph Hickey, both at the University of Wisconsin. "Catastrophic declines of three raptorial species in the United States have been accompanied by decreases in eggshell thickness that began in 1947, and have amounted to 19 percent or more, and were identical to phenomena found in Britain," they declared. The three species were peregrine falcons, bald eagles, and ospreys. They claimed that the eggshell thinning coincided with the introduction of chlorinated hydrocarbon pesticides like DDT, and concluded that these compounds were harming certain species of birds at the tops of contaminated ecosystems.

Still, the researchers just had a correlation between DDT and eggshell thinning. So they did what good scientists should do—they experimented. Joel Bitman at the U.S. Department of Agriculture fed Japanese quail a diet laced with DDT. His study, "DDT Induces a Decrease in Eggshell Calcium," published in Nature on October 4, 1969, found that the quail dosed with DDT had eggshells that were about 10 percent thinner than those of undosed quail. However, Bitman's findings were eventually overturned because he had also fed his quail a low-calcium diet. When the quail were fed normal amounts of calcium, the thinning effect disappeared. Studies published in Poultry Science found chicken eggs almost completely unaffected by high dosages of DDT.

It's not DDT per se that is thought to do the damage to eggshells, but a DDT metabolite known as DDE. Thus the most persuasive feeding study refers to it: "DDE-induced Eggshell Thinning in the American kestrel: A Comparison of the Field Situation and Laboratory Results." This groundbreaking study was published in the Journal of Applied Ecology by Jeffrey Lincer in 1975.

Kestrels, commonly called sparrow hawks, are small falcons. Lincer noted that the "inverse correlation between DDE in North American raptor eggs and eggshell thickness is clear but does not prove a causal relationship since other chemicals or factors could be involved." So to find out what effect DDE might have, Lincer fed captive kestrels a DDE-laced diet and then compared their eggs with those taken from the nests of wild kestrels. Lincer found that dietary levels of three, six, and 10 parts per million (ppm) of DDE resulted in eggshells that were 14 percent, 17.4 percent, and 21.7 percent thinner respectively. "Despite the recent controversy, there can be little doubt now as to the causal relationship between the global contaminant DDE and the observed eggshell thinning and the consequent population declines in several birds of prey," concluded Lincer. As best as I can tell, he's right.

Still, there is a piece missing in the full scientific picture. Despite considerable research, no one has ever identified the physiological mechanism(s) by which DDE causes eggshell thinning, according to Anderson.

There is another possibly confounding issue as well. In 1998, Royal Society for the Protection of Birds researcher Rhys Green published a study in the Proceedings of the Royal Society B which found that eggshell thinning of some bird species had begun 50 years before the introduction of DDT.

There is no space here to outline the entire history of this body of research, so let me direct you to the International Programme on Chemical Safety Web site, which has a pretty good summary of a lot of the research on the toxicity of DDT to birds and other animals.

Those who think I've been duped by the radical enviros on this matter cite the justly famous studies that showed that DDT did not cause eggshell thinning in chickens and Japanese quail. Anderson agrees that the evidence shows that gallinaceous birds (poultry and fowls), herring gulls, and most passerine birds "aren't as sensitive to DDE as raptors." More than half of all bird species are passerine or perching birds, including crows, robins, and sparrows. But even though chickens and quail fed very high concentrations of DDE and an adequate amount of food experienced essentially no eggshell thinning or other reproductive problems, science shows pretty conclusively that it's another story for raptors.

So what elements of my stance on this makes me a corporate stooge—a seemingly contradictory complaint I got from some correspondents? After all, I accept that the scientific evidence backs up the notion that DDT caused eggshell thinning in raptors. But that just shows how cleverly perfidious I really am. By admitting that the bulk of the evidence shows that DDT caused problems for raptors, I give the appearance of being an honest broker of scientific information and thus distract the unwary from my alleged ties to corporate interests. There's no way I can win that one.

So I maintain that it is indeed "generally acknowledged" that DDT thins the eggshells of sensitive raptors. But the enviros won the fight about DDT in America, so why is it still a sensitive political issue today? The main reason is the continuing fight to save millions of people from malaria. Whatever it does to different types of eggshells, DDT remains unquestionably one of the most effective ways to control the mosquitoes that carry the malaria parasite. But international environmentalists have instituted through the UN strict controls on DDT, with an eye on an eventual permanent ban.

Anderson notes that DDT and DDE levels in nature have been falling for decades. Populations of bald eagles, peregrine falcons, ospreys, and brown pelicans have all bounced back. In 1969, researchers reported finding total DDT accumulations ranging from 5,000 ppm to 2,600 ppm in the fat of North American peregrine falcons. Today, one would typically find 50 ppm in raptors, according to Anderson. Such body burdens would yield only about 2.5 ppm in eggs. Anderson notes that there appears to be a threshold of one to three ppm for DDE in eggs below which there is no eggshell thinning in even sensitive bird species. Dusting DDT on the walls of houses in developing countries to control for mosquitoes seems unlikely to cross that threshold for birds.

In Silent Spring, Rachel Carson asked, "Who has decided—who has the right to decide—for the countless legions of people who were not consulted that the supreme value is a world without insects, even though it be also a sterile world ungraced by the curving wing of a bird in flight? The decision is that of the authoritarian temporarily entrusted with power."

Banning DDT saved thousands of raptors over the past 30 years, but outright bans and misguided fears about the pesticide cost the lives of millions of people who died of insect-borne diseases like malaria. The 500 million people who come down with malaria every year might well wonder what authoritarian made that decision.