In an area where DDT had been used extensively, identify the reason why the population of carnivorous birds declined significantly.

Excess use of dichloro diphenyl trichloroethane (DDT) as fertilizer leads to accumulation of this chemical (non-biodegradable) in the birds through food chain, as they are the top predators.
They get the direct effect of this chemical by weakening the shell forming the eggs, as a result the eggs do not hatch as per schedule, which leads to decline of the birds.