In birds, breastbones are modified to hold flight muscles and help in the movement.

Explanation

### Step-by-Step Solution:

1. **Understanding the Structure**: In birds, the breastbone, also known as the sternum, is a crucial part of their anatomy. It is modified to support the flight muscles, which are essential for movement during flight.

2. **Modification of Breastbones**: The breastbones in birds are not like those in other animals. They are adapted to be strong yet lightweight. This adaptation is vital for flight as it allows birds to have the necessary muscle attachment points without adding excessive weight.

3. **Hollow Bones**: Birds possess a unique type of bone called "pneumatobones," which are hollow. Unlike the bones of many other animals, these bones do not contain bone marrow. The hollowness of these bones reduces the overall weight of the bird, making it easier for them to take off and stay airborne.

4. **Importance of Weight Reduction**: The reduction in weight due to the hollow structure of the breastbones and other bones in birds is significant. A lighter body allows birds to achieve greater heights and maneuverability during flight.

5. **Conclusion**: Therefore, the statement that in birds, breastbones are modified to hold flight muscles and help in movement is true. The unique structure of these bones is a key adaptation that enables birds to fly effectively.