A unique feature of birds is

Options

A. homeothermy
B. feathers
C. 4 - chambered heart
D. scales on legs

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for a unique feature of birds. We need to identify a characteristic that is exclusive to birds and not found in other animal groups.

2. **Analyzing the Options**:
- **Homeothermia**: This refers to the ability of some animals to maintain a constant body temperature through metabolic processes. While birds are warm-blooded (homeothermic), this characteristic is also found in mammals and some other groups, so it is not unique to birds.

- **Feathers**: This is a defining characteristic of birds. Feathers are unique to birds and are not found in any other animal group. They serve various functions, including insulation, flight, and display.

- **Four-Chambered Heart**: While birds do have a four-chambered heart, this feature is also present in mammals and some reptiles, making it not unique to birds.

- **Scales on Legs**: Birds do have scales on their legs, but this feature is also found in reptiles. Thus, it is not unique to birds.

3. **Conclusion**: After analyzing all the options, the unique feature of birds is **feathers**. This characteristic sets them apart from all other animal groups.

### Final Answer:
The unique feature of birds is **feathers**.

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