Structures that supplement respiration in birds are

Options

A. lungs
B. air bladders
C. air sacs
D. tracheae

Answer

### Step-by-Step Solution:

1. **Identify the Organism**: The question pertains to birds, which belong to the class Aves.

2. **Understand Bird Respiration**: In birds, respiration primarily occurs through the lungs. Unlike mammals, bird lungs are spongy and inelastic.

3. **Recognize Additional Structures**: In addition to lungs, birds have specialized structures known as air sacs.

4. **Function of Air Sacs**: Air sacs are connected to the lungs and play a crucial role in the respiratory process by supplementing respiration. They help in the continuous flow of air through the lungs, allowing for efficient gas exchange.

5. **Conclusion**: The structures that supplement respiration in birds are the air sacs.

### Final Answer:
The structures that supplement respiration in birds are **air sacs**.

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