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

Options

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, did not hatch

Answer

### Step-by-Step Solution:

1. **Understanding DDT**: DDT (Dichloro-Diphenyl-Trichloroethane) is a synthetic pesticide that was widely used to control insect populations. However, it has harmful effects on wildlife, particularly birds.

2. **Bioaccumulation**: When DDT is sprayed on crops, it enters the ecosystem. It can be washed into water bodies and accumulate in small fish. These small fish are then eaten by larger fish, and the DDT continues to accumulate as it moves up the food chain. This process is known as bioaccumulation.

3. **Impact on Birds**: Birds, especially predatory species like eagles, consume these larger fish. As a result, they ingest high levels of DDT.

4. **Calcium Metabolism Inhibition**: DDT interferes with calcium metabolism in birds. Calcium is essential for the formation of eggshells. If calcium metabolism is inhibited, the eggs produced by the birds may have thin or weak shells.

5. **Consequences for Egg Hatching**: Due to the weakened eggshells, the eggs may either not hatch at all or hatch prematurely. This leads to a decline in the bird population, as fewer chicks survive to maturity.

6. **Conclusion**: The significant decline in the bird population in areas where DDT has been used is primarily due to the impact of DDT on calcium metabolism, resulting in poor egg development and hatching success.

### Final Answer:
In an area where DDT had been used extensively, the population of birds declined significantly because DDT inhibits calcium metabolism, leading to poor egg development and hatching success.

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