I've read that this "fragile egg shell" effect was from a single study in the early 60's where they were testing DDT toxicity on birds by feeding DDT laced feed to ducks. The ducks didn't have any toxic reaction to reasonable levels of DDT, but their eggs were fragile.
It turns out that the analysis methods available in that day required the researchers to feed them calcium free feed, while the control group had normal calcium feed. Naturaly, their eggs were fragile, but initially this wasn't the object of the study, so the difference calcium in feed was not noticed.
Observation of broken raptor egg shells in the wild were seen as proof (go looking for something, and you'll probably find it). However, there is a normal rate of egg breakage, and apparently there were no studies that proved the egg breakage rate was any higher than "normal". They did find DDT in the eggs, if I remember right, but that is hardly surprising, and not proof that it was the cause of the fragility.
Some bird populations did decline in areas of high farm DDT use. But this is undoubtedly because DDT killed their food source, not because of DDT toxicity in the birds themselves.
I hope the article I read on this was accurate. It does none of us any good to spout controversial stuff like this, if it is not factual. However, this seems right in line with the what I've observed of the Green movement. They don't give a tinkers D@mn about people, or even about the environment. They just use it for power, and for a profession, except for the majority "useful idiot" crowd that supports them.
I'm just going from memory, but not only ducks were affected. The hardest hit species were apparently predatory birds. The numbers of raptors are recovering at a very comforting pace - unless you're an envirnazi, because then no matter how prolific a species is, it's 'endangered'.
Most people just figure that the lack of insects were starving the animals that prey on the insects, which then caused starvation in animals that preyed on them.
NOW...because the study results are worthless, I must make the point that we'll never know whether raptors were really affected by the DDT, or some other, entirely unrelated factor. The Greenies and their useful idiots are proof: science and politics NEVER mix. Also, as I pointed out, we have no effective substitute, because ALL pesticides are more and more and more and more closely regulated and then later banned - not because of SCIENCE - but because of pseudoscience, the bastard child of politics and science, which borrows heavily from late '70's, early '80's 'wisdom'.