Osprey numbers declined drastically in 1950-1970s, primarily due to a pesticide called DDT that poisoned fish that osprey ate. Once inside the osprey’s system, the chemical caused the bird’s eggshells to become so thin they could no longer contain the chicks inside. In 1972 DDT was banned, and since then osprey populations have increased rapidly, primarily along coastal areas.
The bird is still listed as endangered or threatened in some states, including Great Lakes states where populations were small or nonexistent after the pesticide years. Osprey numbers also are affected by the loss of suitable nesting trees and habitat destruction for lakeshore and riverside development.