Bald eagles live near rivers, lakes, and seacoasts. The bald eagle does not have any natural enemies. It's enemies are farmers who use chemicals which wash out into the lakes and oceans. People, especially farmers, used DDT to kill bugs. When the DDT washed into rivers and lakes, the fish were poisoned by it.
Eagles eat a lot of fish, and the poison collected in their bodies too.
The DDT made the eagles' eggshells very thin. The shells would crack when the adult eagles sat on the eggs to keep them warm, and the chicks growing inside the eggs would die. When pioneers cleared the land, many tall trees were cleared away, and so were bald eagles. Rainwater that ran over the lands where the chemical had been sprayed was carried into streams, rivers, and eventually oceans. Soon, DDT was built up inside fish.