Once easily seen on the continental United States, the bald eagle was close to becoming extinct because of the use of the pesticide DDT. The DDT destroyed an adult bird's calcium, and it would become unable to lay more healthy eggs. Female eagles laid eggs that were too weak to withstand the weight of its parents. In the early 1700s, the number of bald eagles were 300,000–500,000, but by the 1950s there were only 412 nesting pairs in the United States.