DDT (dichlor-diphenyl-trichlorethylene C14H9Cl5) was a pesticide used on crops to kill mosquitoes. At the time people where afraid of getting malaria from mosquitoes. People thought DDT did not hurt any animals because it did not effect humans, but they were wrong.
DDT was affecting many animals other then mosquitoes. Such as: Bats, Fireflies, Sperrys, Brown pelicans, and of course the peregrine falcon. The birds ate the insects, small birds and fish that contained DDT. Calcium Carbonate is an essential chemical in egg shells. When mother birds ate the animals exposed to DDT, it concentrated or accumulated in her body and caused the egg shell to be formed with a very thin shell. Also, since their shells where too thin the eayes got over heated when incubated or the egg shells would crack before the chicks were fully developed.
Bird watchers and scientists found a decrease in nests and got suspicious. Scientists discovered DDE which is the remaining chemical after DDT breaks down. As soon as it could be scientifically proven that DDT was harmful, it was banned. Rachel Carson's book Silent Spring helped make people want to ban DDT.