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 Bird Migration**: Bird migration refers to the regular seasonal movement of birds from one region to another. This phenomenon is influenced by various environmental factors.

2. **Identifying the Main Controlling Factor**: The primary factor initiating bird migration is the length of daylight. As the seasons change, the amount of sunlight that birds are exposed to also changes, which plays a crucial role in their migratory behavior.

3. **Impact on Circadian Rhythm**: The changes in day length affect the circadian rhythm of birds. The circadian rhythm is the internal biological clock that regulates various physiological processes in organisms, including migration.

4. **Role of Endocrine Glands**: The alteration in circadian rhythm influences the endocrine glands in birds. These glands release hormones that trigger the migratory behavior in response to environmental changes.

5. **Other Contributing Factors**: While the length of days is the main initiating factor, other factors such as availability of food, shelter, and temperature conditions also play a role in migration. However, these are secondary to the primary factor of day length.

6. **Evaluating the Options**:
- Option A: Shortage of food - While food availability is a factor, it is not the primary initiator.
- Option B: Purpose of breeding - Some birds migrate for breeding, but this is not the initiating factor.
- Option C: Search for shelter - Shelter is important but not the main reason for migration.
- Option D: Day length affecting the endocrine gland - This is the most appropriate answer as it directly relates to the initiation of migration.

7. **Conclusion**: The migration of birds is primarily initiated by the length of days affecting the endocrine gland, making option D the correct answer.

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