The exoskeleton of birds is made up of_______.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Structure of Birds**: Birds are unique animals that have adaptations for flight. One of the key adaptations is their lightweight structure.

2. **Identifying the Exoskeleton**: The term "exoskeleton" refers to an external skeleton that provides support and protection to an organism. In birds, this exoskeleton is crucial for their ability to fly.

3. **Recognizing the Composition of the Exoskeleton**: In birds, the exoskeleton is primarily made up of feathers. Feathers are lightweight structures that not only help in flight but also provide insulation and protection.

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