Thinning of egg shells is a major effect in many species of predatory birds because of

Options

A. Natural eutrophication
B. Cultural eutrophication
C. Biological magnification
D. Raduced BOD

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the thinning of eggshells in predatory birds and its cause.

2. **Identifying the Options**: The options provided are:
- Natural eutrophication
- Cultural eutrophication
- Biological magnification
- Reduced BOD

3. **Analyzing the Effects**:
- **Natural Eutrophication**: This is a natural process where water bodies become enriched with nutrients, leading to increased plant growth. It does not directly relate to eggshell thinning.
- **Cultural Eutrophication**: This is caused by human activities that increase nutrient levels in water bodies, also not directly linked to eggshell thinning.
- **Reduced BOD**: Biochemical Oxygen Demand (BOD) refers to the amount of oxygen that microorganisms will consume while decomposing organic matter. While important for water quality, it does not explain eggshell thinning.

4. **Focusing on Biological Magnification**:
- Biological magnification refers to the process where certain harmful substances (like DDT) accumulate in the bodies of organisms as they move up the food chain.
- In predatory birds, DDT is ingested through their diet (prey that has accumulated DDT), leading to higher concentrations in their bodies.

5. **Understanding the Mechanism**:
- DDT interferes with the calcium carbonate (CaCO3) formation in birds, which is crucial for eggshell development.
- A decrease in CaCO3 leads to thinner eggshells, making them more fragile and increasing the risk of breakage, which can lead to decreased reproductive success and potential death of the embryos.

6. **Conclusion**: Based on the analysis, the correct answer to the question is **Biological Magnification**.

### Final Answer:
**Biological Magnification** is the major effect causing the thinning of eggshells in many species of predatory birds.