The migration of birds is initiated by

Options

A. Shortage of food
B. Purpose of breeding
C. Search of shelter
D. Day length affecting the endocrine gland

Answer

**Step-by-Step Solution:**

1. **Understanding Migration vs. Movement**:
- Migration refers to the long-distance travel of birds from one region to another, typically in response to seasonal changes. It is important to distinguish this from general movement, which can be short or long.

2. **Analyzing the Options**:
- **Option A: Shortage of Food**: While birds may move to find food, this typically involves short-distance movements rather than migration.
- **Option B: Purpose of Breeding**: Similar to food shortages, breeding may cause birds to move, but this is generally not the primary reason for long-distance migration.
- **Option C: Search of Shelter**: This option also suggests a short-term movement rather than a long-distance migration.
- **Option D: Day Length Affecting the Endocrine Gland**: This option indicates a physiological response to environmental changes, which is a key trigger for migration.

3. **Identifying the Correct Answer**:
- The correct answer is **Option D**: Day length affecting the endocrine gland. Changes in day length signal migratory birds to prepare for migration, as they respond to seasonal changes in climate.

4. **Conclusion**:
- Therefore, the migration of birds is primarily initiated by changes in day length, which affect their endocrine system and trigger the migratory behavior.

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