In an area where DDT had been used extensively, the population of birds declined significantly because

Dichlorodiphenyltrichloroethane ($\text{DDT}$) is a persistent organochlorine pesticide that undergoes trophic biomagnification within natural ecological food webs. As apex predators, piscivorous and raptorial birds accumulate high toxic loads of $\text{DDT}$ metabolites. These metabolites inhibit the calcium adenosine triphosphatase ($\text{ATPase}$) enzyme within the shell gland, causing eggshell thinning. As a result, the eggs crack easily under the weight of incubating parents and fail to hatch, leading to severe population crashes.