In an area where DDT had been used extensively, the population of birds declined significantly because
(a) birds stopped laying eggs
(b) earthworms in the area got eradicated
(c) cobras were feeding exclusively on birds
(d) many of the birds eggs laid, early hatched
(d) DDT, its breakdown products and other chlorinated hydrocarbon pesticides posed an insidious threat to birds. These persistant poisons enter the food chain and they accumulate in the fatty tissues of organisms at lower trophic level and then tend to concentrate as they move through the food chain. This is called biomagnification or bioconcentration. The insidious aspect of this phenomenon is that large concentrations of chlorinated hydrocarbons do not usually kill the bird out right. Rather, DDT and its relatives alter the bird's calcium metabolism in a way that results in thin egg shells which are unable to support the weight of incubating bird. So, many of the birds eggs laid are early hatched (no chick formation). This causes a significant decline in the population of birds.