How did DDT affect bald eagles?

Answer 1

DDT affected the eggs and made the shells thin and weak. They broke when the mother eagle sat on them to hatch them.

According the the US Fish & Wildlife website DDT was the major contributing factor to Bald Eagle population decline after WWII, by 1963 there were only 487 nesting pairs left - that was the minimum population. Since DDT use was banned in the US in 1972 the population has rebounded even though there is increased loss of habitat & prey for them to feed on (esp. salmon).

The bald eagle population was just beginning to recover when a new threat emerged. The pesticide DDT, which was used to control insects that damaged crops, was getting into the food that the bald eagle ate. The pesticide made it more difficult for the bald eagle to absorb calcium and the lack of calcium made the shells of the bald eagle's eggs thinner. Many eggs broke before they could hatch and the bald eagle population dropped. DDT use was outlawed in the U.S. in 1972, and the bald eagle population has slowly recovered

Alternative answer:

A myth. DDT didn't affect Bald Eagle eggs. Bald Eagle populations reached their minimum in the US prior to the introduction of DDT(1940). Bald Eagle populations were already increasing by 1960 at the height of DDT production and usage.

The real cause of the Bald Eagle's decline was primarily hunting as they were considered a nuisance, a threat to livestock and the fisheries. Secondarily they suffered from a lack of food sources, habitat, poisoning from lead shot and other pollutants (lead, Mercury, etc.).