Birds and mammals share one of the following characteristics as a common feature.

Options

A. Pigmented skin
B. Pneumatic bones
C. Viviparity
D. Warm blooded

Explanation

To determine the common characteristic shared by birds and mammals, we can analyze the options provided:

1. **Pigmented Skin**: This characteristic is primarily found in mammals. Birds do not share this feature as their skin is covered with feathers.

2. **Pneumatic Bones**: Pneumatic bones are hollow bones that are a characteristic of birds. They help reduce body weight for flight. Mammals do not have this feature.

3. **Viviparity**: This term refers to giving birth to live young rather than laying eggs. Viviparity is a characteristic of mammals, as most mammals give birth to live young. Birds, on the other hand, typically lay eggs.

4. **Warm-blooded**: Both birds and mammals are warm-blooded animals, also known as endothermic or homeothermic animals. This means they can regulate their body temperature internally, maintaining a constant temperature regardless of the environment.

Based on this analysis, the common characteristic shared by birds and mammals is **warm-bloodedness**.

### Step-by-Step Solution:
1. **Identify the characteristics**: List the characteristics provided in the question.
2. **Analyze each characteristic**: Determine which group (birds or mammals) each characteristic belongs to.
3. **Find the commonality**: Look for a characteristic that is present in both birds and mammals.
4. **Conclude the answer**: Identify warm-bloodedness as the shared characteristic.