In an area where DDT had been used extensively, the population

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

1. birds stopped laying eggs
2. earthworms in the area got eradicated
3. cobras were feeding exclusively on birds
4. many of the birds laid, did not hatch

High concentration of DDT disturbs calcium metabolism in birds which caused thinning of eggshell and their premature breaking.