Final answer:
DDT causes thinning of eggshells in bird populations due to biomagnification in animal food chains.
Step-by-step explanation:
The pesticide DDT is known to cause thinning of eggshells in bird populations at the top of the food chain. When birds accumulate high levels of DDT from consuming contaminated prey, their eggshells become fragile and prone to breakage, leading to declines in population size. This is due to the process of biomagnification, where DDT concentrations increase as it moves up the food chain.