Adult Female Peregrine
In the late 1940's the application of the chemical pesticide DDT (dichlorodiphenyltrichloroethane) became a widespread practice throughout North America. It's intention was to target pest arthropods that were damaging crops and causing economic losses. What folks did not realize at the time were the environmental consequences that came with its use.
Aside from targeting these "pest" insects, there were many non-targeted species affected. Through a process known in as "bioaccumulation", levels of DDT toxins built up in the fatty tissues of wildlife each time an infested prey item was consumed. With each trophic level increase, the toxins' affect on the animals' physiology was magnified. Many birds of prey, including Peregrine falcons, are at the top of their food chain. Having accumulated such high concentrations of these toxins, the birds were no longer able to properly metabolize calcium. This led to a thinning of the shells that protect their unborn offspring. The eggs broke under the weight of the incubating females. The birds were not able to reproduce and populations were crashing significantly. In fact, they almost went extinct.
Luckily people were able to recognize the damage that was happening. A ban from DDT agricultural use in the USA came about in 1972. Conservation efforts ensued and many of the populations of raptors were successful. Groups such as the Midwest Peregrine Society are a crucial part in continuing the work. Research, Conservation, and Education are their goals. Visit their website here: Midwest Peregrine Society