Andrew, in regard to DDT there are at least two mechanisms at work.
In diurnal raptors DDT interferes with the laying down of calcium as the egg passes through the ovary. This cause the eggshell to be thin - doesn't cause brittleness. The thinness means that the eggshell is weaker than normal, and if thin enough the weight of the brooding parent (birds must warm their eggs for the embryo to develop) will crush it.
Since the ban on DDT, PCBs etc we've seen a steady recovery in terms of eggshell thinness in raptors. Studies I've seen indicate shells in the 95%-ish percent of normal range (IIRC, it's been years since I've looked). Normal thickness is established by measuring eggs collected in the 19th century by naturalists for museum collections.
The second mechanism answers an interesting question, which is the correlation with DDT levels and egg mortality in certain species in which the eggshells are NOT thinned.
Believe it or not, biologists actually look into these things, don't blindly indict compounds like DDT because they're Rabid Leftist Environmentalists or whatever it is folks believe.
In turns out in these species (I forget which order, but it's not Falconiformes, i.e. not hawks, eagles, falcons and friends) the deposition of calcium by the ovary is indeed interfered with, but takes a different form. Eggs are of roughly normal thickness, but investigation by electron scanning microscope has shown that the structure is somewhat "ropy" or "fibrous" rather than a smooth, impervious layer of calcium. The spaces between these fibrous calcium deposits are large enough for water molecules to pass through, which causes the egg white to dry out and the embryo to die.
It yet other orders of birds it appears that the changes in calcium deposition (while real) are relatively innocuous.
I'm just scratching the surface here. I'm posting this in part because I think that many who blindly accept the pronouncements of sources like junkscience.com have NO IDEA as to how deeply phenomena of this sort have been studied, and NO IDEA as to how well-established they are in our current body of scientific knowledge.
Are some environmentalists ignorant of science? Sure. Are some critics of environmentalism ignorant of science? Proof by example: Michael Crichton.
