Which pesticide was banned in 1972 because it softened birds' eggshells?

• A. DDT
• B. Glyphosate
• C. Atrazine
• D. Chlorpyrifos

Answer: (A)

Birds’ eggshell thinning from certain pesticides is the idea this question tests. DDT, a persistent pesticide, accumulates up the food chain. In birds, its breakdown products interfere with how calcium is deposited to form the eggshell, producing shells that are thinner and more likely to crack. That disruption lowers hatching success and helped drive declines in bird populations, which triggered strong public and scientific concern and led to a ban in 1972 in the United States. The other chemicals listed are different types of pesticides with distinct uses and later regulatory histories: glyphosate and atrazine are herbicides with concerns unrelated to eggshell thinning, and chlorpyrifos is an insecticide associated with other health and safety issues but not the reason for the 1972 eggshell ban.